This is how you do it on 2000 so I hope it works for you as well. As always, before perfoming any action that involves registry edits be sure to make a back-up of the registry at a minimum and a system wide backup at an optimum:
Go to the add/remove programs and remove it from there if able and then....
Registry Hacks For Removal Of Cisco VPN Client Software
1) Start, Run
2) Regedit
3) HKey Local Machine
4) Software
5) Cisco Systems
6) VPN Client
7) Delete VPN Client Key
8) Reboot
9) Install updated Software
10) Pray
I would wait a day or so to see if any one else responds and we find out that I'm nowhere near correct on this for XP!!!!