5002 Error then, says Itunes cannot Authorize my account

New Macbook, but I have used itunes for years. When I set up HomeSharing on my PC, everything is fine. I can connect with itunes, the Store, and the computer is authorized.

When I get on the Macbook to start Homeshare, it says I need to authorize, and when I try to authorize it, it either gives me a 5002 error, or it tells me I need to authorize my account. BUT when I get to the store, I can log in and download music, just not play any old music I have purchased.

Please HELP!!!