I was having the same problem but on a Win7 machine. I spent a lot of time looking up sulotions to no avail for me.
My final working solution for Win7 was to enable the local Administrator account, log in, and then install. It wouldn't work under another user account regardless of whether or no i 'ran as administrator'.
As for WinXP, the most detailed solution (although far from a simple fix) can be found at:
http://blog.edwards-research.com/2009/09/ciscovpn-client-32-bit-vista-error-1720/
