ntuser.dat locked after group policy applied(?)

Hello,

We are seeing a problem with users' ntuser.dat being
locked apparently after group policy and virus checker
updates have been automatically applied overnight.

We are running XP Pro workstations connecting to a Win2K
AD Domain.

The first error message in the application event log when
users log in is:
-Windows was unable to load the registry. This is often
caused by insufficient memory or insufficient security
rights.

DETAIL - The process cannot access the file because it is
being used by another process. for C:\Documents and
Settings\'ourusername'\ntuser.dat

There are then three further messages relating to the
roaming profile and the fact that you'll get a temporary
one, etc. etc. Event IDs are 1508, 1502, 1515 and 1511.

It appears that what happens is Group policy and the
McAfee VirusScan updates are applied overnight. The user
will then log back on again in the morning but gets four
errors as above relating to the ntuser.dat being locked.
The only way to release the file is to shutdown and
restart the machine. I have tried to get the users to S&R
in the evening instead of logging out, but that seems to
be beyond them (bless!).

This is only happening on certain machines, and there
doesn't appear to be any established pattern, apart from
if you suffer from the problem, you will continue to
suffer from it. Those unaffected remain unaffected! It is
only happening to some of our 'development' users who are
all set as local admins.

So quick summary:
User logs off and goes home,
Virus Checker and Group Policy updates applied overnight,
User comes in and logs in,
Errors relating to profile occur.
Shutdown and Restart PC, ntuser.dat released, user logs in
no problems.

Is anyone else seeing this? Does anyone have any clues if
it's the Group Policy update or the Virus checker update?
I know the suspicion normally lays with the virus
checkers, so I have disabled that on one of the machines
and am waiting for a couple of days to see if that works.
however, because it's ntuser.dat being locked, I have
suspicions about the Group Policy too.

Obviously we can't disable either of these updates for a
long term solution, so need to find out what is actually
locking the file.

KB searches have so far turned up nothing.

Any assistance from anyone much appreciated. We also have
Win2K workstations running in the same domain and they are
all fine.

Thanks in advance,
Alex.