My PC had Windows 200 and developed a problem where it kept freezing. Not being able to resolve the problem, and assuming it to be a software cause I tried various ways to sort it and ended up doing a reformat and then installing Windows XP. Annoyingly, it still occurs. Can happen any time, but, almost guaranteed that if the PC is left for a length of time, it will freeze up and have to be re-booted. It must be hardware, I've had new fans installed and taken it from the cupboard it was kept in under the desk so that it is now in the open. Still freezing. Getting to my wits end now and thinking of just buying a new PC unless somebody out there can help me out. Oh, just remembered, I also had a new graphics card installed to try and sort it.
And disable the screensaver if you have one. Quite a number of systems from a few years back had problems with certain chipsets when using power saving and/or screen saver options - most commonly when both used together.