said by John2g:
Probably your database is corrupted. Close ZAP and go to Windows\Internet Logs and delete IAMDB.RDB and start again.
just some additional thoughts: yes, this is often due to the database being corrupted. but follow the reboot procedure john2g mentioned (unchecking "load zone alarm at startup" from the overview, preferences tab and rebooting, making sure zap isn't running), and delete not only the iamdb.rdb file, but also the [yourcomputername].ldb file; they BOTH tend to corrupt together. and no need to delete the other files in the internet logs directory; they are mainly logs (though it never hurts to delete them if you don't want to check them again.)
then startup zap, DON'T reset your preferences yet except to recheck "load at startup", reboot, and THEN reset all your preferences, etc.
that should do it. if you still have the problem, post again (and wouldn't hurt to post if it works, too

).
HTH