Can't boot after xp sp2 install

This is a long story so please bear with me,

I can't boot anymore after I installed sp2. I have a home built
athlon64 machine w/ xp Pro. I can't boot into safe mode or nothing. The
machine freezes after trying to load "Mup.sys." and dies with the
error:

STOP C000021A (Fatal System Error)
The session manager initialization system process terminated
unexpectedly with a status of 0xC000026C (0x00000000, 0x00000000). The
system has been shut down.

I've tried renaming Mup.sys and it fails on the driver before that
(vmodem.sys). I don't know what the driver would be after Mup.sys but
commonly (I think) it is agp440.sys which I renamed to no avail. Based
on my research I don't think it had anything to do with Mup.sys or a
driver. In the microsoft KB there is info on STOP C000021A but the
status is not 0xC000026C.

I have not found a solution and I liked the idea of uninstalling SP2
through the recovery console, as per the MS KB. I get there but there
is no $NtServicePackUninstall$ directory!! (I wrote the background at
the end of the post which points out that the machine thought SP2 was
installed succesfully). My question right now is how can that be????
There are many $NtUnistallKBxxxxxx directories with the date stamp of
the night of SP2 installation but no $NtServicePackUninstall$.

Any suggestions on why this is and how can I get rid of whatever
happened (e.g. spuninst.exe each of the hotfixes???)

I'm trying to do everything short of a repair install so any ideas are
appreciated.

The Background:

Initially I installed sp2 through automatic updates. At the end it
complained that the installation failed and it had to uninstall the SP
(that "items ... manifest ... verification ... failed ..." crap). So it
uninstalled and reported that "Windows may not work properly."

I got lucky and windows booted and seemed to work fine. I researched
this and found many recomendations to avoid the automatic updater and
downlaod the full (272meg) SP2 file and install from that. Stupid me
(slap myself), insisting on installing sp2, went ahead and did that
(download + install). It said that the installation was comleted
successfully. Now I can't boot at all anymore (safe mode, vga mode,
last known good mode, whatever.)

I have tried removing both cdrom drives, and my crappy (not MS
certified) modem. I don't have integrated video so I can't remove the
NVIDIA video card. I'll try removing the sound card tonight.

Enough rambling from me and thanks for your time.