Hi,

Yes, overheating usually causes that, but it would be more of a time based thing, and related to usage rather than an application. Graphics intensive applications may provoke it.

BIOS can cause strange things, but not a virus scan shutdown.

So, I suspect that there is a problem with the files, and I have seen this sort of thing before, only it froze and forced a manual reboot rather than actually shut the box down itself.

1. Run chkdsk, then defragment.
2. Reboot into safe mode.
3. Run the virus scan, but change the scan rules to ignore compressed files.
4. If that works then reboot into normal mode, reset to scan compressed files and scan again.

If that crashes then you will have to search for compressed files and open them individually until you find the corrupted one.
You might look in the system logs to see if there are any clues.