I too have had the same problem, and for 2 known reasons, both different occassions....
First, the Hard Drive was overheating, this was happening regardless of when, where, or how long I was on the computer for....it was exactly how
clownfish describes it.
and
Second time was when we installed anifra-red optical mouse...the computer played up with fatal exceptions, froz, unfroze, and generally didwhatever the hell it wanted to the entire time the mouse was installed...You can imagine our embarrasment when, after having taken it to be repaired for the 3rd time with no joy and finding out it was a blasted conflict of the mouse software,and my hard drive
Don't know if either of these are your problem, but it just goes to show how many really silly and seemingly inconsequential things can effect the running of your PC.
Good luck with it!!
GG
