Been pulling my hair off all day trying to have Cisco VPN Client work from my Windows 2003. Was getting an error about Reason 412: the remote peer is no longer responding. It was working fie from Vista, but not from Win 2003. After a zillion of suggestions found in internet that didn't work, the solution came to be very simple. So if anyone will have this issue again here is the solution:

Open the Cisco Profile file (it's a file ending with .pcf) in some editor (Notepad is good enough) and add the following line to the [main] group:

UseLegacyIKEPort=1

Save, close and restart the VPN client. This should do the trick.