Change Default Username

Not so much a functional problem just a niussance... here's the story,
I have a windows xp pro box that logs in to a domain at a clients. They
recently had some turnover and a new employee is using one of their
computers. When the computer boots up, the ex-employees username is in
the login box by default. No problem, just type in the correct one and
password and it works fine, but I'd like it to have the new employees
name by default instead of the ex-emplyee.

note: I'm not sure if the username was just changed or if the account
was removed and the new one added, this was my first thought, but I'd
like to have another solution if it was indeed removed and a new
account created.