adding domain user to local admin group

Hello. I have run into an issue and am in need of some assistance. I am managing a local domain for one of my company’s project locations. I have had a new employee come to the sight and have added him to the domain. The OSs are Windows 2000 server and 2000 WS. My co-worker needed to be able to install software on his laptop so I tried to add him to the local admin group of his WS. While going through the process of adding his domain user to the admin group it asked for a domain admin username/password, I entered both. A list showing all of the users for the domain appeared. When I clicked on his username to add him to the local admin group (WS) the following message comes up “Processing of object failed with the following error: The specified domain either does not exist or could not be contacted.” Any help would be much appreciated!

Thanks!

Ron