I already knew on Monday night that I had a problem, but I didn't want to think about. Sure enough, on Tuesday morning my computer was misbehaving. Rather than bore anyone with the pain and anquish I went through trying to work around the problems--let me just say that I had backed everything up, so even if my computer had completely died it would not have been a catastrophe--I'll skip to the solution. At some point, I realized that what I thought was a virus protection program that Microsoft was trying to force me to buy was actually a virus that my Norton software was doing battle with, turning my screen into a bloody mess. So I Googled "XP Antivirus 2008" (the virus I had) and got more information, and learned that few standard protection packages could combat it effectively. This pointed me toward a free program, SuperAntiSpyWare, which I downloaded, updated and ran. No more XP Antivirus.
Windows appears to have been a little damaged in the process, but all other functions appear normal, so . . . back to work.