ntvdm.exe consumes 99% of CPU

Hi, I am running XP Home (actually re-installed a month ago from scratch) and find that if the PC is left unattended for about an hour the ntvdm.exe process starts up and slows everything down as it consumes 99% of CPU load. It never starts on initial boot, only after significant unattended time.

I have run Virus scanners and Spybot to see if it clears it, turned off screensavers, etc. but can’t find out what is starting it.

Can anyone give me some tips on tracking what is setting ntvdm.exe running, ie. can I use Process Explorer or similar to trace it?