Graphics card(?) causing crashes

I've got a Geforce GTX 660Ti

It's been running fine for 5-6 weeks. Today I launched Left4Dead2 and got 7-8 FPS (usually a rock solid 300 even at the busiest moments).

In case of driver/software issues, I reinstalled Steam, applied all outstanding windows updates, reinstalled the graphics driver (clean install) and finally (after validating local files) reinstalled L4D2.

Now, I've got a slew of problems...

• Launching a 3D app (Anything vaguely modern) causes an immediate crash
• Viewing a site which uses WebGL works fine (http://lights.elliegoulding.com/ works fine)
• GPU-Z works fine (GPU load < 5%)
• Nvidia System monitor crashes after a couple of seconds
• L4D2 crashes immediately
• Tropico 4 crashes immediately
• The longest journey runs fine
• Skype works fine until I turn on my webcam, then crashes immediately
• VLC crashes after 10-20 seconds
• Win7x64 aero glass works fine

Looking at the event viewer, provides a number of problems associated with the crash...

Skype

Faulting application name: Skype.exe, version: 6.6.0.106, time stamp: 0x51c414b3
Faulting module name: KERNELBASE.dll, version: 6.1.7601.18229, time stamp: 0x51fb1116
Exception code: 0x80000003
Fault offset: 0x00013219
Faulting process id: 0x748
Faulting application start time: 0x01cec5fcd7006ca4
Faulting application path: C:\Program Files (x86)\Skype\Phone\Skype.exe
Faulting module path: C:\Windows\syswow64\KERNELBASE.dll
Report Id: 73d21ec0-31f3-11e3-abde-1c6f65cec842

L4D2

Faulting application name: left4dead2.exe, version: 0.0.0.0, time stamp: 0x52262a0b
Faulting module name: tier0.dll, version: 0.0.0.0, time stamp: 0x52262908
Exception code: 0xc0000417
Fault offset: 0x00022264
Faulting process id: 0x23cc
Faulting application start time: 0x01cec60004dfe146
Faulting application path: D:\Games\Steam\steamapps\common\Left 4 Dead 2\left4dead2.exe
Faulting module path: D:\Games\Steam\steamapps\common\Left 4 Dead 2\bin\tier0.dll
Report Id: 475ab7b2-31f3-11e3-abde-1c6f65cec842

Nvidia Monitor

Faulting application name: NVMonitor.exe, version: 6.2.13.1, time stamp: 0x48a9a323
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0xc0000005
Fault offset: 0x0000000200000000
Faulting process id: 0x1200
Faulting application start time: 0x01cec5feb3eb4ce7
Faulting application path: C:\Program Files (x86)\NVIDIA Corporation\NVIDIA System Monitor\NVMonitor.exe
Faulting module path: unknown
Report Id: fbfc9425-31f1-11e3-abde-1c6f65cec842

How can I diagnose what is causing the problem? Is it definitely a graphics card problem?

GPU-Z

So.... Turns out that when developing an app which uses DirectX, I flipped DirectX into debug mode to track down a problem. Debug mode gives detailed information when something goes wrong, but it is extremely slow and many optimizations are disabled.

Since I'd forgotten to swap it back, some apps (OpenGL based) ran perfectly, some ran slowly (L4D2) and some just failed entirely, presumably because they were relying on an implementation detail that was different when debugging.