you didnt supply much info on this but i will try to help.
in what way are you trying to send the mp3 file to your friend? if your trying to send it by email what client are you using? does it only happen with mp3s?
have you tried adding the mp3 to a zip file then sending.
if that fails there is always file transfer at www.yousendit.com
this site allows you to send files to a server that then emails your friend with the link to download, files can be up to 1gb so is good with big files, depending of course on your internet connection.
hope this helps
everytime i try and send a mp3 file to a friend computer trys to reboot.??????