If you do not follow these instructions when installing the Cisco VPN Client on Windows 7, you will probably end up in an endless blue screen / reboot cycle!
Uninstall any previous version of Cisco VPN client that you may have installed
Download the Citrix DNE Update and install it.
Make sure you have Cisco VPN Client 5.0.4.0300 (or newer, if available)
Install the Cisco VPN Client
source : http://weblogs.asp.net/bhouse/archive/2009/01/15/how-to-successfully-install-cisco-vpn-client-on-windows-7.aspx