In this Microsoft article, "Troubleshoot Windows XP Professional", it states the following:
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If you are unable to join a domain:
Make sure your computer is physically connected to a network.
Make sure the domain name you used is correct.
Make sure a computer account exists for you.
Verify that the DNS server and domain controller are running and online.
If none of these steps help solve your problem, join a workgroup instead, and then join the domain after installation.
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The entire article may be found at this link:
http://www.microsoft.com/windowsxp/pro/using/howto/gettingstarted/guide/troubleshoot.asp
(REMOVE SPACES from the pasted URL.)
It's a quite lengthy and very thorough article with several additional related links.
Good luck,
Maxwell