Michael,
In all honestly - why did you upgrade? What extra funciontality does 5.x give you over 4.8 on XP???
Read the below link:-
http://www.cisco.com/univercd/cc/td/doc/product/vpn/client/5_0/50client.htm
But I will para-phrase:-
"The VPN Client release 5.0 introduces support for Windows Vista"
Ver 5.x has so many bugs I personally will not be a beta tester for it, if I ever have to upgrade there will have to be some very good reasons to do so.
4.8 is the most stable latest version going - downgrade.
Just a little note - upgrading when you don't have to, just for the sake of it = head aches.
HTH>