[Samba] NT Domain login problem

[Ninti Systems]

I have set up an NT domain using samba 3.0.4 on Slackware 9.1 with Win
XP clients. Security = user, samba is PDC and WINS server.
I'm new to samba and working most things out as I go, but have one
persistent problem:

Some clients frequently refuse to log in to the domain, a situation that
can only be remedied by, for example, logging in as local administrator
on the XP machine, then logging back out and trying to log in to the
domain again (usually successful the second time round).

Is there some setting or configuration step I might have missed?

Thanks
Mick