Issues upgrading from XP home to Pro re: AD domain

I do some side work for a small company. They have 3
computers that when purchased were running XP home.
Somewhere a long the line, someone upgraded them to XP pro
so they could connect to an Active Dir domain.

For the most part, the computers work fine and ARE able to
authenticate with the domain. Here's the problem I am
having...

I sign into any one of the three computers with the domain
administrator account, go to Computer Manager \ Local
Users and Groups and try add a domain user account to the
local Administrators group (or any group for that
matter). However, when I click Add, and then click
the 'Look In' list box, only the local computer is
available. It does not see the domain. I can then log
off the domain admin account, and log back onto the
computer using any domain account I wish.

I think there may be something hanging around from XP home
that is preventing Pro from being able to see the domain
when assigning group permissions. My question is has
anyone heard of this happening? Also would anyone know
how I could resolve this with out having to do a format /
reinstall of Pro?

Thanks in advance!

Rusty