Unintalled SP2, restored Windows from SP1a, Windows Update fails

I was having difficulty with a USB ADSL modem that wasn't working, and a
colleague recommended that I remove SP2 from my XP machine. I was dubious,
but did as they suggested. The uninstall hung for several hours at one point
but kept going. Unfortunately when I rebooted Windows as instructed by the
uninstaller, the machine failed to restart.

Eventually I had to use my OEM CD to repair Windows back to SP1a.
Everything, apart from Windows Update, seems to be working okay; but WU dies
with the 0x800A01AD problem.

I've tried re-registering the msxml*.dll series; I've applied the upgrade
patch to msml v4; I've checked the registry values for Windows Update and
they're correct; I've tried stopping & starting the WU service... I've tried
everything that's been suggested here on the newsgroups and in the other
WU-recommended resources, and nothing works.

I assume the "repaired" Windows I'm now running is in some indeterminate
state between SP1a and SP2 which Windows Update can't extract me from.

What are my options? I presume I could perform a complete Windows reinstall,
but I'd rather not wreck the registry and have to reinstall all of my
applications...

Ah - found one suggestion that *does* work: downloading the full SP2 patch
and reinstalling it.

"Paul Harman" wrote:

> I was having difficulty with a USB ADSL modem that wasn't working, and a
> colleague recommended that I remove SP2 from my XP machine. I was dubious,
> but did as they suggested. The uninstall hung for several hours at one point
> but kept going. Unfortunately when I rebooted Windows as instructed by the
> uninstaller, the machine failed to restart.
>
> Eventually I had to use my OEM CD to repair Windows back to SP1a.
> Everything, apart from Windows Update, seems to be working okay; but WU dies
> with the 0x800A01AD problem.
>
> I've tried re-registering the msxml*.dll series; I've applied the upgrade
> patch to msml v4; I've checked the registry values for Windows Update and
> they're correct; I've tried stopping & starting the WU service... I've tried
> everything that's been suggested here on the newsgroups and in the other
> WU-recommended resources, and nothing works.
>
> I assume the "repaired" Windows I'm now running is in some indeterminate
> state between SP1a and SP2 which Windows Update can't extract me from.
>
> What are my options? I presume I could perform a complete Windows reinstall,
> but I'd rather not wreck the registry and have to reinstall all of my
> applications...