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?
I'll try to say it as succinctly as I can: When I try to play videos, they come up looking... well, here's the best way I can describe it--they look like the gamma is at full and they've been horribly JPEG'd. So far the only way I have been able to make them play like they're supposed to is in WMP 10, by turning off overlays in video acceleration. I don't have any viruses or spyware, I've tried RealPlayer, QuickTime, and Nero Showtime with the same results, and the problem occurs with every type of video media (except I can't make DVD's play right). Somehow, it worked before, but I'm not sure when and how. Does anyone recognize my symptoms? I've got an NVIDIA GeForce FX 5200 graphics card with 128M, Intel Pentium 4 3ghz, 2G RAM, running Windows XP SP2 on a Sony Vaio PCV-RS712.

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