Does anyone use this? I found Prime95 to be the best stress test of the CPU for stability. I don't think the "95" in the name means Win 95 since it runs under any Win OS.
Anyway it calculates an uber prime number to 400 iterations. It reports any result errors it encounters in a log. What is neat is that it can detect very subtle CPU errors before any of the typical symptoms appear (like Windows errors and crashes to desktop).
Say you recently did some overclocking and you very occasionally experience a crash to desktop, maybe once in every 8-12 hours or so, you can't be sure if its a hardware problem or maybe the drivers you just installed, or maybe its just a buggy game... In situations like this prime95 is ideal. Just run the stress test over night, if it gives you no errors and no warnings then your CPU is stable