VPN on WinXP

We are utilizing a Cisco 3005 VPN Concentrator (formerly known as an Altiga box) for our remote access requirements. For all previous OS (Win9x, WinMe, Win2000) the default VPN client works perfectly. On all of our installations of WinXP, the default VPN client will not connect to our concentrator. We are not using IPSec, only PPTP and L2TP with default configurations on the VPN box.

We are basically at a loss as to why Windows XP default VPN client is not connecting. We have discussions started on MS web sites as well, but as it is no surprise, MS has been no help in resolving this issue. Any thoughts?