For some reason I cant play some mp3's I have downloaded in windows media player.

IT comes up and says "Windows Media Player cannot play the file. The file is either corrupt or the Player does not support the format you are trying to play."

Seems to be only after I have played them on the build in player

May I ask what is the file type?
maybe the file type is new and WMP doesn't recognise it. Also you might want to try downloading the latest version of WMP.

That's a typical response from WMP when it doesn't have the correct codec to play a particular file (audio/video.) There's different types of mp3's out there & some require special codecs to play. You'll need to obtain these xtra codecs. Also there's mp4 (m4a) which is a new type of format that's showing up. WMP will not play these. Winamp 5.02, Apple iTunes, Compaact!, Nero 6, dBpowerAMP, Foobar2000 are audio/burner apps that have the ability to play m4a files. In summary, I'd suggest you obtain xtra codecs for WMP or/& get a hold of xtra audio app (WinAmp is good.)

Yeah, that happened to me for some of my mp3's. So i would suggest putting the songs in itunes or realone player. These play every one of my files without a problem. If you don't have itunes, it is really easy to download. Just go to the apple website. You can download it for PC or mac.