The only new thing that I've downloaded is the XP SP2.
Well this could be completely unrelated but this is getting interesting. I've also installed SP2 recently and I'm getting a blank screen on occasion. It only happens if I leave the computer for an extended period of time, similar to you.... The only way to get the video back is to reboot. Other than the above (recent) problem the computer seems rock solid.
I'm going to disable the screensaver (Marine Aquarium 2) to test if there is a bug in SP2. If that doesn't work I'll try to roll back the SP2 installation. I'll let you know what happens.
BTW, I'm using a 9800 Pro card and the power management settings are set to- ALWAYS ON.