Did you use windows media player to rip your music? have you used it toedit/manage your mp3 collection?
if so check your settings, it could be ripping tracks to the WMA format, and adding a 'security' feature that protects copyright.
in other words Windows media player adds (can add) something called DRM to music that stops it being played on anything other than the authorised machine/player.
your best solotution is to turn off rights management or whatever its called, and re-rip your music, into MP3 ( or another DRM free format)
if its stuff bought online then you should be able to play it on mp3 players that support DRM infected music.
If its from the napster subscription service, then only certain portable players support the playback of those files - while your subscription is current, see the subscription service website for more info.