The server 2003 installation (which was set up long before I arrived) has severe problems with profiles, DNS settings and other things. I won't bore you with the details, but suffice to say that starting from scratch with it seems the best option (it's a small network - about 30 users).
My one niggling doubt is with how this will affect Exchange. My plan is to change the groups/users which were created in a fairly random ad hoc way. Will it cause problems trying to associate the new users with already existing exchange mailboxes (i.e. to avoid having to start from scratch with the Exchange server)?