It could be the screen saver (try another one), but I don't really believe that.

Other ways to go:
1. What are the settings for energy saving in Windows (Config Panel)? What are the settings for energy saving in the BIOS? Can you notice any time (say 30 minutes, 1 hour) which corresponds to your computer freezing?

2. Heat. Some screen savers (or video processors) can heat the CPU by showing you the most intricate pictures. The best screen (and money, and your and my environment - if you believe in the effect of CO2 on the climate) saver: just a black screen.

3. Heat. Check the fans. Try it with the case off.

4. Power. CPU and video use power. If PSU is getting older it might not be up to the task. A new power supply might help. However, see #2.

5. Parasites. Rather unlikely in this case, I should say.

Kees