Mac OS X 10.5.7 update breaks Cisco VPN Client
After updating to 10.5.7 I started to get kernel panics and the system would not start.
To fix this, restart your Mac in safe mode by pressing shift. Then launch the command line and use this to uninstall the VPN client:
sudo /usr/local/bin/vpn_uninstall
This will remove the kernel extensions which prevent the system from booting up.