Hey, I found a fix for you
here.
Related to XP and Vista.
I'll rewrite it in an easier way...
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1. First, make sure Windows Media Player is closed.
2. Go to Start > Run > Type in
services.msc and press OK.
This will show a list of Services.
3. Scroll down and look for services starting with Windows Media Player.
4. Right click on each of them and choose Stop. Close Services.
5. Go to Start > Run > Type in this and click OK:
code:
%USERPROFILE%\Local Settings\Application Data\Microsoft\Media Player\
6. Highlight everything (CTRL+A) and delete them.
Now when you restart Windows Media Player, you should be able to add music to your library again.