KB925902 update causes STOP:c000021A fatal error at boot

My PC automatically downloaded update KB925902 yesterday. With the update
installed, the PC would boot into a BSOD with "STOP:c000021A {Fatal System
Error} The Session Manager Initialization System terminated unexpectedly with
a status of 0xc000026c (0x00000000 0x00000000) The system has been shut down.

None of the start up options would get the machine booted. It's running XP
Home SP2 with all prior updates installed.

In the end, I got the Recovery Console up and running and removed the
KB925902 update by running 'batch spuninst.txt' This removed the BSOD problem
and allowed the machine to boot OK.

So, the machine's now running but I'm without the KB925902 update and have
disabled Automatic Updates to prevent it installing for the time being.

Two possibilities seem to exist:

1) There's a problem with the KB925902 update which needs to be fixed.

2) There's some other incompatibility on my machine which is causing the
BSOD when the KB925902 update is applied.

Any thoughts please? I gather elsewhere that the STOP message above is
indicative of a security or permissions problem with system processes
towards the end of the boot.

Most likely. Do you have overclocked your machine? Or "tuned" otherwise?

Hm, which applications/services are running in the background and
were running while applying the update?

Do you have looked for updated system drivers (BIOS, chipset etc.)
for your system?

An update...

I've just called Product Support Services.

They tell me that the KB925902 update is incompatible with my XP Home SP2
and they recommend that I hide the update to prevent its installation.

I can't see the sense in this. KB925902 was issued to fix a security
vulnerability which I would then still have, so this doesn't fix the problem.

I'm running Windows Vista premium and I can't even install this update.

I noticed your searching and posting about this issue on the web... and
finally came across your solution..

Many thanks, as this stupid update took down my windows 2003 server... Not
wanting to lose a whole week of clients data, I was so glad to see this
solution..

I thank you... for saving my server...

I only wish Microsoft would own up to issues like this one. How an update
can take down systems is beyond me.

Interesting is that this kb925902 update only killed one of my 2003 servers.
Of course it had to kill the one that had the most important info...

Why are you only now installing a security update from 2007?

The corresponding KB article mentions an issue with some printer drivers which
can cause a system crash, but nothing about failures to boot. If they were
common, it is likely that the problem would be documented.

In more general terms, when an update causes a system to fail the most common
causes are an already existing malware infection or faulty device drivers or
other third-party kernel-mode components (such as antivirus software).
Unfortunately there are too many possible combinations of hardware and software
out there for Microsoft to test updates on all of them.

(Once in a while the update really is at fault, of course, in which case
Microsoft usually withdraw the update until it can be corrected.)