Through the fickle hand of providence, I have just now found the problem--a bug in the NVIDIA something-or-other software that sets the gamma range to high. Two more things, though:

1. For those who have Googled here: to fix the problem, right-click on the desktop, go to NVIDIA Display, select your monitor. Color correction. Apply color changes to: All. Color profile: Advanced mode. Apply.

2. The color is back to normal now (a little dark, actually), but the JPEG-like image defects which the gamma increase brough into sharp relief are still there. I tried to play a DVD, and the resolution looked terrible. Is this a related problem? I could fix it, as I said before (not in DVD playback), by disabling overlays in WMP 10... and I know that for a while, I could watch DVD's perfectly fine. Right now I am playing DVD's with no decoder installed (according to the "Video Decoder Checkup Utility")--could it be that I need to buy a decoder? Why does this problem appear in videos (any format) unless I have overlays turned off? And why does it persist for DVD video when I have overlays turned off in the "DVD video" section?