I can recall that getting into XP Safe Mode was a PITA.
I would disable all startup tasks and do a normal restart - if I can add books the culprit is a startup task, if not rinse and repeat with Scheduled tasks..., if not rinse and repeat with Shell extensions..., if not rinse and repeat with 3rd party Services..., if not
Once you've identified the group do a binary chop. I've seen people to spend days trying to guess the cause of a problem, which they would have solved in an hour if they'd tackled the problem methodically.
BR