Fatal error C000021a upon XP update?

As described by other users with the same problem, the machine (runs XP home)
worked fine prior to updating automatically and displayed a fatal error
message when rebooting. The machine doesn't start either in safe mode or with
last known good configuration.

As suggested in this forum and elsewhere, I uninstalled the lastest updates
(installed just before failure) with the recovery console by running the
uninstall.txt for updates #931768 and 930916 (one at a time). The machine
still doesn't boot. I do not know whether I should uninstall prior updates (3
weeks ago) even though they didn't cause the machine to fail booting up. Any
other suggestions?

The error message is the following:
STOP: C000021a {fatal system error}
The session manager installation system process terminated unexpectedly with
a status of 0XC000026c (0X00000000 0X00000000). The system has shut down.

If anyone is interested, here is the follow up as of now. With the recovery
console, I replaced the system32/config files with those found in the
windows/repair folder to try booting up with the initial config but that
didn't work. I searched the net for fixes that applied to my error message
(C000021a with status of 0XC000026c) but couldn't find any; in fact,
Microsoft KB doesn't have a page on the topic even though there are people
who experienced that type of crash before upon updating. I disabled
agp440.sys as it seemed the cause of many similar fatal errors but that
didn't do anything. I eventually resigned myself to do a clean reinstall
(considering the amount of time I had already spent on this) but the install
CD doesn't recognize any prior OS and thus doen't allow the repair option
which'd result in installing a 2nd version of XP. Finally, I tried rebuilding
the boot.ini file with bootcfg in the recovery console but failed with that
too! So, it looks like I'll have to rebuild the machine from scratch but
first, since my wife is a bold artist who works without a net (it's her
machine), I mounted the drive as a slave on a different XP box to back up the
data ... and I'll be ready to proceed. Of note, I did a clean install of XP
SP1 and subsequent needed updates on this machine about a year ago.