microsoft.public.windowsxp.security_admin

So heres the thing:
I have Windows XP Pro (Out of Waranty Dell pre-install) normally fully
patched up within a few days.
4 Users (+unused admin account) 1 user has a password so I can use Remote
Desktop. 3 Users have no password set. Welcome Screen and Fast User Swiching
enabled.

Now and again we get to to the welcome screen and no matter which user we
click it asks for a password and even the right password on my account fails
to log in. No-one can log in.

Solution is to reboot or to connect and disconnect a remote desktop session.
Then normal service is resumed. The later is a one time fix - the next time
we see the welcome screen it needs to be done again.

Not exactly sure what sequence of events gets us to the problem situation
but quite
possible that running a remote session is a pre-requisite. Also happens
frequently when one of the no-password users screen saver kicks in.

Its been going on for months and have failed to find a solution searching
the net, although some other references to what sound like similar problems
also unsolved. Would raise an issue with MS but reluctant to part with real
money.

System should be clean, symantec AV running, other diag tools run from time
to time with nothing ever reported.

Suggestions and corroboration most welcome.
Thanks.
Stu.