Delete the local profile for that user and log'em back in... Unless that profile has tons of stuff in it... You could also try re-running the internet access or is it remote access wizard again so that IE reconfigures itself.
Now I have a question for you... Is it really an NT domain or is it an AD domain? Cause if you are still running an NT domain for shame! But since you logged on to his computer just fine it is mostlikey a profile issue. Copy out his My Doc and Local Mail store (If not using an "always on" exchange connection.) And any other folder like their desktop... The strange thing is that if they were a person that was moved to a new domain it "should" setup a new profile like username.domain in the Documents and Settings folder. But if they were not moved and still sitting in the old one... Well, I don't know exactly what to tell you... But them again I am one of those that need to see it to give you my best answer/fix.