explorer.exe deleted repeatedly

I have a problem with a Vista machine which started while viewing a youtube video (ave maria andrea bocelli if it matters). Several messages appeared faster than the operator could process them and the machine shut down. On restart the desktop would not load.

Explorer.exe was not available in the Windows directory so I loaded it onto a flash drive, ran it from the Task Manager, and copied it to c:\Windows.

When I restarted it was missing again.

I used CCleaner to clean up the startup, to remove old files (very few) and registry entries, and copied explorer back the the Windows directory again.

When I restarted I had the same problem with an absent explorer but also when run from the flash drive windows explorer would no longer open any folders.

Advice will be appreciated.