A strange, very hard way to stress test your PC:
I have been doing this for a couple years now, works great, as long as the system doesn't lock up because of lag (it isn't uncommon)...
When I need to see what my PC can do, I run the most hardware intensive games I have. And lots of them. And I play them all via Alt-Tab (or, for Vista, an added stress is the Flip 3D, where you have the game actively playing as it scrolls through).
Right now I am running:
Windows Live Messenger
Test Drive Unlimited
All 3 games are on their highest settings (except resolution, I do this windowed) and I am getting about 2-3 FPS in Test Drive Unlimited (8-10 in Halo 2, and about 30 in Halo 1).
I am running all this, and if I don't see a performance drop, I will continue opening games.
Now, off to check my email in Windows Live Mail...