try to do a registry restore from the command line. That is if you can get to the command line. or check this from micro shaft
http://support.microsoft.com/?kbid=307545
But in a nutshell remember this
1) The WinXP registry consists of 5 files located in
c:\windows\system32\config directory
SYSTEM; SOFTWARE; SAM; SECURITY and DEFAULT
You could choose to copy or back these up separately from Ontrack or
SystemState.
2) It is possible to extract these five files from any valid system restore
point by the following means:
In windows explorer go to the folder called 'system volume information'
there you should find one or more folders _restore{*********}
This folder will contain subfolders called RPxxx, each containing another
sub folder called 'snapshot'.
In this folder the registry files are called:
_REGISTRY_MACHINE_SYSTEM
_REGISTRY_MACHINE_SOFTWARE
_REGISTRY_MACHINE_SAM
_REGISTRY_MACHINE_SECURITY
_REGISTRY_USER_DEFAULT
Knowing these files/folders, if you can't start Windows, you can use DOS
commands to restore any/all in FAT32 and Recovery Consul in NTFS (renaming
them as per (1) above).
May be useful sometime.
You should be able to use the above info to go backwards before you did the update.
Hope it helps!