Strange Logon BSOD

[TigerWolf]

After installing Firefox 9.02 (i am not sure if this is what caused it, i recently installed Office XP but that had a successful restart) i now are unable to use my computer as when it goes to logon stage, i get this message.

STOP: c000021a ( Fatal System Error)

The windows logon process systems process terminated unexpectially with a status of (0x00000080 (0x000000000 0x000000000)

Also, new updates from windows update were also installed

Note: WinXP PRO, SP1, installed from Unnatended installation disk only recently

The computer compleletly shuts down, only leaving the above message on screen

And now for the question: How do i fix it? How do i stop it in the future? What is it?

[TigerWolf]

Update

*Safemode doesnt work as it must use the logon module aswell

*Last know good config doesnt work either

*Will try to repair with WinXP cd

So now the question is, how can i access all of my files (so i can backup onto second drive in computer) so that i can format again, without using the screwed windows (im thinking about trying a repair from the windows cd, but i dont like my chances of it working). I need to take into account its on an NTFS formatted drive, so this might prove tricky. Is it possible to get windows on a CD (bootable, like knoppix for linux) or install another copy of windows on the same drive? and i really need to find ou t what the error is so i can prevent it happening in the future.