NTP time synch works manually, fails when logged off

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When I use the Network Time Protocol (NTP to synchronize the my Windows
XP Pro machine manually with time.microsoft.com, it always works. When
all users are logged off, but the machine is running, it always fails,
with the following warning in the error log:

Source: W32Time
Type: Warning
EventID 36

"The time service has not been able to synchronize the system time for
<large number> of seconds because none of the time providers has been
able to provide a usable time stamp.
The system clock remains unsynchronized."

The problem is repeatable.

The problem isn't due to the machine hibernating -- I can do remote
logins.

I have my DSL gateway firewall and Windows firewall both configured to
permit NTP, and it always works when I do Adjust Date/Time / Internet
Time / Update Now.

Any ideas how to debug the problem?

Answers to the following questions might help:
Which process does the NTP stuff?
Does Windows Firewall behave differently when no user is logged in?
Is there a log somewhere that gives additional information?

.

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