This morning I noticed that, when I hovered my mouse pointer over items in the Taskbar, I didn’t get the usual miniature preview windows. Instead I got a set of small dark blue windows with the names of the documents that were open by the indicated application. I’d never seen anything like that before. Then I noticed that the top parts of all the windows had lost their translucency. I recognize the translucency as an Aero feature, and figured that for some reason, Aero was broken. I hadn’t gotten to that right away because I’d never seen Windows 7 without Aero.
I tried a few things without much success, and then I had a thought. I went into the AMI Catalyst program, turned off 30 bit color, and rebooted.
Problem solved. I did some research; it’s a problem with Windows 7, and the NVIDIA documentation notes the incompatibility with their graphics cards as well.