Why does my PC crash only when my cat is nearby?

It might be another thing the root cause of your problem, and the cat behavior AND the computer shutdown to be both the effect of this cause. Lucky you, the cat's sensitivity threshold is better, and you can anticipate when your computer will stop just seeing the cat's behavior who is acting in advance. Maybe the cooler from your video card is starting to make a noise (which cat can hear, but humans don't), and this makes the cat curious and goes to check what's there. Or maybe the humidification is too high, and the cat feels anxious and is searching for a better place in the house (just behind your computer). After that, the protection from your computer triggers for the same reason.

Theory 1: Misplaced ATX standoff

I’ve built and diagnosed computers for years, and this sounds somewhat like an incident I helped a customer with once.

The customer reported that computer worked fine, but if the computer was bumped (or someone with static electricity got near the tower), it would reboot.

Upon closer inspection we discovered that the brass standoffs that connect the motherboard to the case had been misplaced, which caused one of the standoffs to be touching the back of the motherboard.

Theory 2: Rear IO shield short circuit

This is rare now that many motherboards have integrated IO Shields or use shields with a thin layer of foam. However if your motherboard has an all metal IO shield that has any metal protruding parts, it's possible it could be touching something that it shouldn't be. I've mistakenly made this error several times on PC's I've built, only to find the metal tab is wedged in the RJ45 or USB ports.

Admittedly these are both rare incidents. However your symptoms are also rare and suggest some combination of static electricity and a short circuit.

When I've encountered problems like this in the past, the fastest troubleshooting has often been to completely disassemble the computer and then rebuild it. In many cases reseating all the parts fixes the issue. In rare cases you find something that shouldn't be touching something else.

Lastly as others suggested, ensure you are using a grounded electrical outlet and possibly try a new power cord if you have one.

I would start by placing four dry sponges beneath the box. If the box is plastic I might try to paint the inside with that conductive paint that people use on guitars for some reason. Then I might turn on the computer and then remove as many cables as possible without making it inoperable. Finally, I would turn it on and remove everything but the power cable and wait for the cat to come around. This is all based on the presumption that static electricity is leaking into the box somehow. Electricity around delicate components can cause all sorts of weird things. Is your electric power in the house stable? Any defective equipment generating EMF, such as faulty electric motors that the cat can hear? Isolate possible causes and you will narrow them down.

TL;DR

It was most likely caused by static electricity, the fact your computer can be affected by it means it isn't properly grounded and shielded, however this will be nothing more than a hypothesis unless you take action and do the experiments to verify it.

Reasoning

An animal's fur rubbed against plastics holds positive static electrical charge, though you didn't say what fabrics the carpet is made of, presumably it's plastics, and the cat is an animal, therefore the cat holds positive charge after it has walked on the carpet.

Good conductors, such as metal, can't build up static electricity as the charges are quickly dissipated away, cats are made of conductive material, but cats are poor conductors because of high resistivity, therefore cats can build up static electricity.

It's easier to build up static electricity in Winter because of low humidity, the air is dry because most water vapor is condensed into liquid because of low temperature, water is a good conductor unless it is distilled, if the air is wet, it would be harder for cats to build up static charge because the charge will be quickly dissipated away.

Electricity is one side of electromagnetism which is one of the four fundamental interactions, every macroscopic force that isn't gravity is electromagnetism, electromagnetism doesn't require contact to take effect because it is carried by electromagnetic fields, a field is a matter that spreads through space-time, its strength is negatively correlated to the square of distance to its source, so theoretically your cat can cause your computer to crash when it's close to your computer while it is charged.

How your computer is affected

Computers like most electronic products are susceptible to electrostatic discharges:

If we have one object that has a really high positive charge and another object that has a really high negative charge, the charge from the object with the higher electrical potential flows or jumps to the object with the lower potential. That discharge can damage electronic equipment. Computer components such as memory chip, CPU, motherboard, hard drives, and expansion cards can be damaged by an electrostatic discharge as small as 100 volts or less. 100 volts sounds like a lot, but ESD discharge must be about 3000 volts before we can see it or feel it. That means that we could be damaging computer components with smaller voltage which we don’t see or feel. Damage can occur simply by placing a fingertip too close to a component inside the computer case. ESD can cause immediate failure of components, or could gradually degrade components, causing only intermittent problems. To suppress ESD, we can keep the relative humidity between 40-70% and temperature between 72-77 degrees Fahrenheit (22 to 24 degrees Celsius). We should avoid dry air in the computer repair location.

Source

There can be three causes:

1. Your computer touches the carpet

   As you said in the question, the computer pretty much touches the carpet, since the cat holds positive charge then the carpet holds negative charge, then by extension your computer holds negative charge, therefore it can be affect by ESD.

2. Your computer isn't grounded

   In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.

   Source 2

   Ground wire protects your computer from ESD because it dissipates any static charge that builds up to the Earth.

3. The casing of your computer isn't metal

   Metal shields your computer from static electricity:

   A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. Faraday cages are named after scientist Michael Faraday, who invented them in 1836.1

   A Faraday cage operates because an external electrical field causes the electric charges within the cage's conducting material to be distributed so that they cancel the field's effect in the cage's interior. This phenomenon is used to protect sensitive electronic equipment (for example RF receivers) from external radio frequency interference (RFI) often during testing or alignment of the device. They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.

   Source 3

Possible steps to verify the theories and solutions

For cause 1, put your computer on dry wood and away from the carpet.

For cause 2, use a three prong plug and three prong outlet, and make sure your house has the ground wire, contact a local electrician if you have any doubts, you might need to rewire your house.

For cause 3, use a metallic case.

I appreciate that there are a lot of useful diagnoses of your problem here, but in the short term, and assuming that your room has a door:

Keep the cat outside the room!

I can only speak from a UK background - other countries may also have doors, please check your local building codes. :)

Does your cat have a magnetic collar? Many cat flaps come with magnetic collars for your cat which acts as a sort of "key" for the flap, so other animals can't get in. These magnets can be quite strong, and the magnetic field could be interfering with your PC (this would be more obvious if you had an old-fashioned VGA monitor, it's harder to tell with modern screens).

If you've already tried wiggling the cables then static-proofing comes first but I'd still be highly suspicious of cables.

Static proofing means a low impedance connection from motherboard to case to a proper ground. That's all you can do without major PITA. If you built it yourself then you may have loose earth straps and sub-standard components. If a screwdriver doesn't work then you're stuck. However a good earth connection should be a minimum requirement regardless of static issues. Wiggle for continuity between case and plug (obviously with the plug out of the wall socket.) The next bit of checking the house wiring is good (quite possibly it is horrible) is a specialist subject. You can get fancy plugs with lights that'll tell you, but perhaps best get an electrical professional. Ignore that for now as there are much simpler things to do. Do you actually have a static problem? Rub your feet on the carpet and earth yourself on one of the case screw heads. Take off a polyester jumper and try the same.

Anti-static spray. Back in the 1980s we used to spray a weak mixture of fabric conditioner and water on the carpets around fancy computer stuff. TBH for your tower PC it's best to switch it off and avoid spraying in the vents.

My guess though, is cables and distribution blocks. In the last 6 months I've had a 'perfectly good' mains cable in exactly this situation go BANG. It took ages and a lot of 'it must be the PSU' etc) to find a pinhole wound on the third time! It was shorting in the middle of plain 3-core cable and blowing the fuse! My solution would be wiggle each inch of the cables to start with to see if you get a re-boot. Then cable-tie everything together (like this https://www.youtube.com/watch?v=-SGk0ohzx1M ) and keep it up out of cat's way.

Simplest of all get, a cardboard box the cat can just fit into, shove it near, but not on top of, fairly tidy cables and everyone is happy.

Follow the comments on https://www.theregister.com/2021/02/12/on_call/ for all manner of weird reasons for this sort of thing.

UPDATE

I googled the bugcheck and according to first few links it seems like GPU problem. It could be driver problem or overheating problem.

Suggestions which I found:

1. Try to upgrade GPU drivers to the newest one or if you already have it and crashes continues, try to downgrade it because the crashes could be caused by a bug in the newest driver.

2. Try to decrease temperature of GPU by removing dust from cooler, decreasing room temperature etc. Also put the computer on hard and flat surface (table, piece of wood, glass, metal etc. ), then fresh air will have better access to the case also it stops sucking the dust from carpet.

3. The problem can also be caused by McAfee or Norton products. If they were installed on your current system then it could cause the problem. Find how to remove them completely or make clean installation of Windows.

You can also check if dumps are saved when BSOD appears. For Win7 its: right click Computer -> Properties -> Advanced system settings -> tab Advanced -> group Startup and Recovery -> Settings check if you have checked "Write an event to the system log". For other systems it should be similar.

ORIGINAL

Have you checked logs/dumps, if it is caused by software? It is possible you connected something to your computer and crashes are caused by the device. Or software/firmware updates could cause it. Or any part of computer could be made with defects which appears after few days/months of using device.

The cat in your room could have a bad luck so it seems that he is doing that.

If you want to be sure that it's caused by static, then make sure that your cat is not in the room, close doors/windows to make sure that he won't appear near. Then take sweater or anything else you know that it can cause static shock and create the shock.

If nothing is problem (even from other answers) and your computer still crashes when the cat is in the room then be more time with your cat because he got bored and now he is learning how to use telekinesis.

Try removing the connection between mainboard and the case's reset switch. If your mainboard is badly designed, the electric field surrounding a fully charged cat might be sufficient to trigger the mainboard's reset circuit.

Not a proper explanation per se. Just a personal experience to support many other answers here saying that it is caused by static.

I have Samsung SyncMaster S23B350T Monitor in front of me right now. Sometimes it goes dark for a few seconds and then restores when I get static shock while getting up from my chair. And sometimes it even does this when one of my colleagues is getting up from their chair a meter or two from my desk. The monitor has external power supply connected through a barrel jack and is isolated from main's earth. Maybe this is part of the problem.

So it's quite probable that your problems are also caused by static charge building up on your cat's fur.

-- EDIT --

I've just realized that my previous statement that the monitor is not connected to main's earth is incorrect. It is not connected via power supply. But it is connected to PC's case through shield of HDMI cable. And the case is grounded.

I should also probably note that I have two monitors connected to the PC. And I never have such problems with the second monitor which has internal power supply and is connected to main's earth directly via power cord.