When your mp3 file has a Windows Media Player icon, it has been connected to WMP.
If not, it will not open automatically in Media Player.
You can right click on your mp3 file, click Open with, click Choose program, click on Windows Media Player, and click on Always open this filetype with this program (guess that's probably the translation from Dutch XP).
Another last way is to open Windows Media Player, go to extra, options, filetypes, and select mp3. Press apply and then OK. Your mp3 files are now assigned to WMP.
Anyway, those options will do exactly the same as Hengis' way.
(I'm not quite sure about my English :roll
