Join a AD domain with the FQDN

Hi

I have a AD with Windows 2003 R2 that was already configured by another person. I didn’t know how he installed it but it seems something is wrong, because I cannot join the domain from a newly installed XP box using the FQDN of the domain (office.mycompany.com), I can only join the domain using the pre-Windows 2000 name (OFFICE), but once I’ve joined, in the “computer name” properties of the XP box I can see the “office.mycompany.com” domain.
How can I fix this?
I think the problem was that in the beginning the DNS was not configured on the domain controller. Now we’ve just migrated it to the DC, everything works but this. Any idea?