I'm using Windows 98SE, but this may not work with other systems.

Go to Start/Settings/Folder Options and click the File Types tag.
Then search down the list for RealPlayer.

If it's missing (you removed it, didn't you?) type "N" and backtrack to
"MP3" on the list.  Then simply edit this item to tell the computer
what program you want it to open for mp3 files.

Regards,
John Page (UK)