If you copy purchased song files between computers, then the first time you try to play the song on another computer it'll ask you to authenticate using the iTunes store login/password that purchased the songs. Then it will authorize that computer to play the songs. You're limited to 3 computers simultaneously authorized, so you have to choose the "De-authorize this computer" option before you reformat or sell the computer, etc.
I just convert everything in MP3 format and stick them all on a shared drive in my file server. That way everyone has access to them and you can play them in whatever software and on whatever device/computer you want.