Well, what do you know.....I have a laptop with the same issue, but with Vista. Of course, the client isn't giving any info except "the computer keeps restarting all of a sudden".
System will seem to boot into Vista, but just before the logon screen the system will reboot. Safe mode, normal, etc. all the same. Halted reboot on system error and received the c000021a bsod. Googled the error - plenty of results, very light on solutions, even from Microsoft.
I tried scanning the drive with Dr. Web Live CD - no go. Definitions load, but the scan never starts. Very curious. So, I remove the hard drive and slave it in another system. Several trojans were found and removed. Reinstalled hard drive - same error.
So, now I'm thinking of performing a repair installation. Very curious that I cannot access the Administrator account, just the main user account (I receive an error when trying to access the Admin account). While I'm there I decide to use the recovery console to execute sfc /scannow. That doesn't work - receive error: Windows Resource Protection could not perform the requested operation.
By the way, I did scan hardware for issues, reseated/replaced ram & ran a bios update. System passed all hardware tests.
What I'm thinking is that the system was hit with a rootkit or trojan that changed system permissions, deleted all restore points and may have changed some system files. I also feel that someone who really didn't know what they were doing tried to fix it and made it worse. I don't know if this system has (had) SP1 or SP2 installed and at this point I really don't care. I already made an image of the drive and backed up all data (first thing I actually did), so tomorrow I'll be performing a n&p (first one in a while). I feel defeated, but sometimes ya gotta do what ya gotta do.