A friend of mine has had this really weird freezing problem recently and I'm not sure what is causing it. When hes just browsing the net and doing stuff like that, occasionally (not sure how often but its not infrequent) he gets a blank screen for a few minutes or it just freezes on him. Now this may not seem to strange, but the thing is that after hours of gaming, the problem never shows up. Only in windows. He formatted only a few months ago so I dont think viruses are causing it but ill make sure he scans for them anyway. He has fairly recent drivers too.
Specs of that system that I can remember off the top of my head are:
a64 3000+ (s754)
1HDD I think
couple of fans
TT 430w psu, not a fancy one tho
Well because it's in windows and not when the system is under stress it is most likely software.
Or just plain chance. If it was completely random then I'd say maybe HDD/ram/psu. You could run a HDD scan. You could try swapping ram from another computer to see if the problem continued. You could monitor the voltages using any number of programs available. If you see dips and spikes that could be it but it would be more likely to occur under load so I doubt it's the psu.