I installed the software firewall GlowWorm on my G4 Powerbook running OS X 10.5.1. It hosed my system something fierce; don’t do it. Everything froze, and I had to cycle power to reset the machine. Subsequently the machine would not restart normally. I had to reboot in safe mode ( first time ever) and then uninstall GlowWorm. Fortunately, there was an uninstaller in /Library/Application Support/GlowWorm/utilities.

BTW, safe mode took about 15 minutes to come up, so give it time if you find yourself in this situation.