Time sync failure... why?

I am running Windows XP SP2. I need to figure out why I am getting an
error whenever I try to synchronize my clock with an Internet time
server, either automatically or by clicking "Update Now."

I initially assumed that Norton Internet Security was interfering with
the process, but I tried disabling its firewall, its intrusion
detection, and its spyware detection, without results. Norton's logs
do not indicate that it has taken any action when the time sync fails.

When I try to synchronize with time.nist.gov, I get the message "An
error occurred while Windows for synchronizing with time.nist.gov."

When I try to synchronize with time.windows.com, I I get a similar
message which sometimes has additional text, saying something like
"The error was: operation failed because the client's stratum is below
the host's stratum." I don't have the exact wording because this
message is not appearing consistently, and I can't get it to appear as
I write this.

I have looked at the suggestions in the Help and Support Center, but
none none of them appear to be relevant. There is nothing wrong with
my Internet connection; I have tried disabling the firewall; the
problem cannot be up the server because I get an error consistently
with both servers and any time of day; and my computer's time is set
very close to the correct time (within seconds). Something else must
be happening, but what?