I have been sharing media between my Xbox360 and my PC for some time now with no problems what so ever. Two Weeks ago I noticed that my library in Windows Media Player 11 had disappeared and a day or so later My Xbox could not detect my PC anymore. When I try to add any files to the library nothing happens and if I go to Media Sharing it gives me this MESSAGE "Media sharing has been turned off because a required Windows setting or component has changed" I have TRIED all the web help options that the Microsoft website offers such as restarting the WMP sharing service. After countless WMP11 uninstalls and reinstalls  along with system restores to before the problem started I am hopeless. If anyone has encountered this problem before or has any idea of why this is happening please reply. thanks!Remove WMP 11 and put things back the way they were. I have not tried this myself, but other people on other forums are saying that it is the FAULT of the new WMP update. 
As for me, I am USING WMP 6.4 and have no plans to update!  thanks geek but i tried that and no cigar. and anything below WMP11 does not allow sharing. Does someone else have access to this machine regularly ? ?I had this same issue.  UNINSTALLING and reinstalling will do nothing.  I tried fooling with Windows Media Player on another user account on the same PC and had no issues, so I just created an additional account, titled Media.  I set up my library within Windows Media Player (11) and was able so successfully share my media with my 360 once again.  For S and G's I gave it an XBox 360 theme and now have my music and movies available on the 360 again.