Local User keeps disappearing after reboot

I need to create a local user on a client PC, for a software package to run. I do this either at the pc or remotely say something like ?davetest? after a reboot the newly created user is gone.
We run a windows 2003 server domain with the client PC?s (which are all the same make and model). The clients run XP pro service pack 2
The PC?s are setup by RIS image from the server (if that makes any difference).
Is there something in Group policies which could be doing this?
I have run ?gpresult? on a test machine but I can?t see the problem.

Any ideas
Many thanks
Dave