|Never heard of exactly that issue.|
I guess it could be a hard drive or memory issue.
I doubt a virus is doing it but it could be possible.
If it were me, I'd run some diags maybe. Vista has memory diagnostics and that may be a good first test just for because.
The only for sure way would be to run tests or do a clean load from OEM disks. It may be time to do that anyway. Then get some good A/V software, even free one's like Microsoft's Security Essentials is good.
Why did it take me over a year to phone in a problem to ATT?