Strange… Are you logged in with Administrator privlidges, or just as a user?
I do not know if any files in said directory are essential to windows, or if windows might complain… but you could try accessing the folder via command prompt
(start->run->type “cmd” and hit enter.)
To get to the windows media folder, at the command promt, type (with quotes):
*when you see [enter] it means to hit the [enter] key, not type “[enter]”
cd “C:\Program Files\Windows Media Player” [enter]
hit enter, Your prompt should look like the first pic.
dir /s /b [enter]
*for info on each command, type “command /?” without quoutes. Example: dir /?
Do you see any file names? (like second pic…although I don’t have WMP 10).
Often you can do [access/delete/rename/overwrite] things (both good and bad) that don’t work in windows explorer. I do not suggest you try to delete/overwrite anything in the WMP folder, as I do not know if: (1) it will work, or (2) will screw up windows even more. You could see if M$ has a offline (stand-alone) installation file for WMP. You could also try to locate a CD (mabey a game or something) that has a WMP installer on it, and try running that.
If WMP is not requires, I would suggest MPC (Media Player Classic 220.127.116.11), or VLC (Video Lan client). Other players include TCMP, BSPlayer, and Zoom Player.
Links to download the above at http://www.videohelp.com/tools?s=11#11.
I hope this is of some help.