Okay, here goes:
1) Try to install version 11. (Or did you do that already? It's not clear.)
2) If that fails try re-installing WMP10, and immediately see if it will uninstall. Then in Add/Remove Programs -> Add/Remove Windows Components, try to revert to WMP9
3) If that fails, do the above in reverse.
4) If either of the above works, run CrapCleaner on the registry. Free:
http://www.ccleaner.com/
Hit "Issues", back up the registry when prompted, and then fix problems.
5) Get CodecSniper. Free:
http://www.videohelp.com/tools?s=38#38
Post a screen shot or list the ones it tells you are "broken". CAREFUL about deleting any.
6) If any need to be deleted, do so only if you're certain what they are, or get the okay, then clean the registry again.
7) Get GSpot and/or AviCodec, from the same page as CodecSniper, both free. Load a file that doesn't play, it will tell you the specific codec you're missing. Google for the codec. Install that one only.
8) Codec packs cause a lot of people grief. Sometimes nothing can be done but a format/reinstall. The infamous Nimo Codec Pack was notorious for that (curse you Nimo!). If one is determined to install a codec pack, the basic KLite pack is, to my knowledge, safest.
9) Tried this: Start -> Run -> sfc /scannow (?)
All I can think of right now. [scratches head] Good luck.
