When Windows refuses to boot, it makes it far more difficult to fix this kind of error. I assume you've already tried to boot into Safe mode with minimal drivers (i.e. hold F8 down in the early boot process). Without Safe mode working, you'll have to use your Boot CD.
I suggest you first try a earlier restore point, but I doubt it will fix it. For XP, instructions are at: http://www.faultwire.com/solutions/using_system_restore_xp.php
It's more likely you'll need to re-install Windows. Instructions are available under Solution #4 at: http://www.faultwire.com/solutions/fatal_error_BSoD_solution_guide.php
Look for the sub-section: Reinstalling XP without erasing settings and applications