I ran Secunia Software Inspector and amongst other things it reported real player was insecure and posed a secutity threat so I followed the link and downloaded Realplayer 10.x another was Adobe Reader which I have also updated to Adobe Reader 8. That has fixed the insecurities but now all my music is in realplayer and I cannot remember how to set windows realplayer back as default player also my son cannot now access a network shared music folder could that have anything to do with the changes made when I downloaded realplayer 10. I did a system restore to before downloding realplayer but then I couldn't open any web pages, window opened sying this page is unavailable bla de bla so restored back to after the download and all is fine on the web page issue.
Yes when you installed Real Player, it would have given you the option of configuring your .mp3 files with that player, thus removing WMP as the main player for these types of files.
To change:
Right click on an .mp3 file. click Open With then Choose Program, A pop up window will appear, click on Windows Media Player and then select ALWAYS USE THE SELECTED PROGRAM TO OPEN THIS KIND OF FILE, click OK