When I try to sign into MSN messenger on my Xbox (Which I've just got back from repair) I'm told "This is your first time signing in to Messenger on this console. Please enter your Windows Live ID password."

It then takes me to the password screen, I put my password in, then it takes me straight back to the "This is your..." screen. There's no like "Wrong password" or anything.

I've tried changing the Windows Live account associated with my Xbox, but it just says "Xbox Live is unavailable. Please try again later." But I'm signed into Xbox Live!

Anybody had this problem or know how to fix it?
Humph! I'm not getting a reply from anyone at Microsoft... Is there anybody at all that can help me?