Last time I start Windows there was an error and then a blue screen appeared. It said "STOP: 0x0000003b" and it forced me to shutdown my PC. It said that something might be wrong with some software or hardware.
This error has been associated with Firewire issues in Windows 7. Can you specify Windows version?
MS has a Hotfix for the FireWire issue
Have you updated any drivers recently?
Have you booted and tried Last Known Good? That may help.
Does it happen often? Is this the first time it happened? What were you doing before it happened?
You should be able to find a mini (kernel memory) dump file in C:\Windows\Minidump.
You can use NirSoft BlueScreenView to view the dump files and to try to pinpoint the cause of the problem.
You can download it here:
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