Windows XP Pro & NT 4 Domain

Hi,

I have a Windows XP Pro w/SP1A on a Windows NT 4.0
Domain. We have groups created on the domain(XPAdmin)
that we add into the local groups on the PC
(Administrators).
If a user logs onto the PC and has user only user rights.
But then is added to the Domain Group(XPADMIN) that is
part of the Administators group on the local PC. Their
creditials are not updated so that they have Administator
rights on that PC.
Does anyone know why? Or is there a fix?
Initial I am thinking that in Group Policies in MMC there
is an option there as to the number of cache logins (10
default) does this need to be lower? If we set this at 0
will this also not allow them to log in if the domain is
down?

Thanks... Tony