Service Pack 3 worked!! For the record, the latest python is also necessary. Thanks!
Linuxcoin: I use it on another rig but this one won't boot a usb stick. Just donated today--I love it.

Well, it worked for 10 minutes, then I got a popup ATI error report (saying the card was no longer responding), requesting to send it. Hashes showed like it was going but no activity in the control panel and low temps. So I restarted guiminer, it went for 2 seconds, then the computer powered off.

And not in a gentle way. I am letting it sit for a bit.