Windows XP won't let me login after installing Personal Firewall.

I just installed the latest Personal Firewall on a Windows XP Media Center box. I rebooted the box, and was presented with my login screen. I type my password, and it starts loading my settings, then says "saving your settings" and drops me back at the login window. I can't seem to get in. I rebooted into Safe Mode, which showed me my userid AND Administrator on the login screen. They both do the same thing. Has anyone ever heard of such a thing? My next thought is to boot from a Windows ultimate Boot disk, and try to find the Personal Firewall Executables, and rename one or two - perhaps it will bypass with errors and come up.