From Control Panel Add or Remove Programs, you can remove MSPlayer. It gives you the option to back up to the previous version (if it is still on your system). Go down the list of Add/Removes; it is listed as 'Windows Media Player 9 or 10'.
I had a problem with RealPlayer. It quit working when I tried a trial RA2AVI program to convert RealPlayer files. The RA program installed features of RealAlternative. RealAlternative replaces some RealPlayer files and RealPlayer will no longer work. Even after doing the uninstall, I had to do some utility searching and removing manually of a bunch of RealAlternative files. RealPlayer 9 and 10 would try to install, but would stall out. I finally had to install RealPlayer 8, which worked and said there was an updated version (10). I let it update, and now RP 10 is working fine.
I'm not sure why you lost the capability to play wmv files. WinAmp has a new version out. You might try it. Also, your associations might have changed (if wmv's open with another player). You might try updating the MSPlayer. I inadvertantly got 10 loaded but uninstalled and went back to 9.
[I thought I lost the capability to full-screen movie files. But 10 and 9 loaded with defaults checked. I found in an info forum that unchecking 'Enable full-screen mode switch' returned the full-screen capability.]
Windows Media Player is not in my Add/Remove Programs. It WAS in the Add/Remove Windows Components, but I removed it and it still worked. I checked the Program Files, and it was not removed and I was able to double-click a video file, WMP10 loaded, and I got the Codec error.
This happens with files that used to work, but suddenly don't work anymore. I've reinstalled WMP10 a number of times, but that doesn't do the trick.