Windows RAS, Netscreen Firewall and Windows Native VPN Clients

Hi,
I want to set up a VPN using Windows RAS that will allow remote
clients to have access to the network. So far I have been able to
connect to my RAS server from the local network and it seems to be
working okay. But I cannot get this traffic to be tunnel through my
Netscreen firewall.
I wanted to channel PPTP traffic to a virtual IP mapped to my RAS
Server but there is no option for choosing PPTP as a virtual IP
service.
I am using Netscreen ScreeOS 3.0 and WIndows 2003 Server as my RAS
Server and my remote machines are running Windows XP and Vista. Is it
possible to set this up?