"Paul" wrote:
HKLM\SYSTEM\WPA\PosReady === New key
Installed DWORD 1 === New DWORD value

Putting that in, as far as I know, I was part of the
administrator group. But when I tried to remove it,
the XPMUser account could not remove it, I elevated
to SYSTEM using psexec and that didn't work either.

I think the reason is that the system process has a handle open on
that key (as it does for all others under WPA). You could try closing
the handle first but then the OS might panic.

The only level left in my collection is TrustedInstaller,
and I wasn't going to bother with testing that.

AFAIK, XP doesn't have TrustedInstaller.

The Registry is a file system, and the entries have
permissions, and doing it from Linux, the expectation
is the permissions will be ignored.

It's not a permissions issue. I own the PosReady key as an admin and
have full control. I also have full control of the parent.

If you leave the PosReady key, it just means that
Windows Update lists a lot of stuff that may or may
not be appropriate as a patch.

I've not noticed any unsuitable patches or updates and no new ones are
being offered. There have been a couple of problems so they may be
exceptions. One update was repeatedly offered despite failing to
install. It was a multi-processor kernel update not relevant to my
system. I had to block it in the end. The other was something that
changed ownership and/or permissions on the registry hives for the
local system and network service accounts which prevented them being
used. The OS still booted but had to create temporary directories and
files for those accounts with a bunch of errors in the event log. Of
course, being originally an XP Home edition, I didn't have access to a
file permissions dialog in explorer to correct things. I had to mess
about with Powershell to sort it out.