Why can't I update XP anymore?

I can no longer update XP. Any idea what might be wrong?

When I go to Windows Update (http://www.update.microsoft.com/
windowsupdate/v6/default.aspx?ln=en-us), click on Express, it tries to
validate my Windows system. That fails. Why!?

It gives me the option to try an "alternate validation". That
works(!). But when it throws me back to the window and I click on
Express again, it again tries to validate my Windows system the
original way, which fails again.

Things I have tried.... Disabled Norton Internet Security. Reset IE6
Security and Advanced options to their defaults (medium level
security). In IE6, cleared history and deleted cookies and files
(including offline content). Added www.update.microsoft.com,
update.microsoft.com and windowsupdate.microsoft.com to Trusted Sites
(all http://...). Restarted the system after all of those changes.
Accessed Windows Update via Start >> Programs to ensure that there is
only the one IE6 instance running.