Does it really hang? Is the little colored bar still moving in the login box?
It may be that your computer is attemting to start a service/get a dhcp address/register it's dns, or something that requires domian admin access.
If the bar is still moving, give it a few min (no more than 10) and see if it gets any further. If it does end up logging in, look in the eventlog when it comes up and check for delayed services/programs.
Do you have any special login scrips assigned to the domain admin acct that might be trying to do something on login?
When you try to login as domain\administrator, can you go to another workstation at the same time and try to access the servers shares/remote management?
For a test you can create a new domain user, add them to the domain admin group and see if that user gets any further (when doing this start from scratch, don't copy an existing user)
If that works, then maybe the profile for domain admin is currupted. make sure no services are using the domain admin acct, Reboot the server (needed if you've previously tried to login ad domain admin sometimes), Using another admin acct (local or remote) Move (don't delete, you can go through steps to recover it later if you need to) the C:\Documents and Settings\Administrator.Domainname folder to somewhere else. The folder might be named something else on your server.
Then upon next login it will recreate the profile. you can then copy some of your settings back from the old folder.
-Teva24