Unable to join domain

Ok, so the first question to ask is, "Do you have at least 1 Windows domain controller running?" Let's assume so.

Ok, if you have Windows 2000 Domain Controllers with Active Directory, then make sure your XPPro client has, as its primary DNS address, the IP address of one of your Domain Controllers that is running the DNS Server. XP uses DNS first by default to find the DNS records of the Win2K Domain Controllers.

Now, if you still have NT4.0 PDC/BDCs instead, then you will probably needto use the LMHOSTS file to get XP to find the PDC. You would just add 2 records into the LMHOSTS file on the XP box: one record associates the IP address to the machine name of your PDC; the other record points the NetBIOS name of your NT4 domain to the IP address of your PDC.
For instructions on how to work with the LMHOSTS file, go here:
http://support.microsoft.com/default.aspx?scid=KB;en-us;q180094
(please remove any spaces)

hope this helps

To add a couple of things to Answer 1:

When joining an AD-aware client (Win2K/WinXP) to an AD domain, you must enter the entire AD domain name (yourdomain.com). When joining a non-AD domain (WinNT), you need to use the NetBIOS domain name (YOURDOMAIN).