First of all, Windows vista home will not let you join a domain b/c its not designed for domains. Second, if you are the administrator of the domain, you can log into Vista as a user but when you try to join to the domain, you will have to enter your Administrator credentials or the credentials of a user with admin rights. Windows XP and Vista (business class) are very DNS specific and will freak out if you don't have the proper DNS Server associated with the machine. Be sure that you have the Active Directory Domain controller listed in the network properties of the client machine as the dns server. Also be sure that you can ping the AD server as well. If you can, you shouldn't have a problem.

If you need more help with that, feel free to email me.
cheard@cs3services.com
Thanks,
Chan Heard