Remote Desktop connection

I am able to make a Windows VPN connection to my network through my Cisco PIX firewall and then make connection to my Win 2003 servers via remote desktop and to my Win NT servers via VNC. I can also make connections internally between Win XP machines using either remote desktop or VNC. But I cannot remotely make a connection to any Win XP machines through the VPN connection using either remote desktop or VNC. Any suggestions?