hmm, I don't recall having too many problems when I joined my Vista machine to the server 2003 box. I set up the computer, and I created an account on the 2003 box with permissions to add computers to the domain. Jumped back onto the Vista box, went to add the computer to the domain, and it asked me for a login, and all was well after that. You could dig through the security policies to see if there's anything preventing it, but I wouldn't know where to tell you to start with that.