Since this one was somewhat esoteric I thought I'd share it.  Searches 
online hadn't come up with much in the way of helpful information.  Or 
perhaps I should say *successful* information.  One page finally gave me 
something that I played with to success.

Had a Vista computer that was displaying stop code c000021a on bootup, 
even when booting into safe mode.  Actually it was displaying a quick 
flash of blue and then restarting, but that handy option in the F8 menu 
for disable automatic system restart took care of that.   (Now that the 
system is up and running again I've disabled the auto restart)

There were a number of system restore points going back some months.  
All of them failed and could not restore.  Trying to capture an image of 
the system for a backup using imagex also failed. (I can't explain that 
one myself)

Disk scans come back clean, full hardware diagnostics come back clean.

The problem had something to do with the software hive file of the 
registry.  Not sure precisely what, I had even tried removing everything 
from the run & runonce areas with no success. (A fun new use of WinPE 
for me is using regedit to open the hive files of a system so you can 
work on them even without being able to boot.) 

If you can replace the hive file with an older one you can fix the 
computer in 30 seconds.  Happily windows may even be able to supply you 
with one in the regback directory.

Load up WinPE or your boot environment of choice and do this:
CD c:\Windows\System32\Config
copy Software Software.bad
copy Regback\Software Software  (answer yes you want to overwrite)

Restart and *poof* the problem goes away. 

YMMV.  (Your mileage may vary)

Hopefully this will help someone at some point.