Can’t add a domain user to a local group.

I have a window XP Pro sP3 PC that is being reassigned to a new user, and what I normally do is add the new user to the Local Administrators, Remote Desktop, and Antivirus groups. When I add the AD user object to the group, I get the window stating that it can’t find the username, and to remove the object. I tried adding several other domain users to several groups on the PC, and none of them work.

Any ideas?