You'll wanna use DNAT in the nat table of iptables
<exlb> = external ip address of linux box
<vpn> = vpn or pptp port (usually 1723)
<winnt> = ip address of winnt box
iptables -t nat -A POSTROUTING -d <exlb> -p tcp --destination-port <vpn> -j DNAT --to-destination <winnt>:<vpn>
What this _should_ do (you may have to experiment) is take any packet coming to the <exlb> destined for port <vpn> and changed the destination ip address of the packet from <exlb> to <winnt> and send it on it's merry way. Make sure the <winnt> box can send packets to the internet.
Make sure /proc/sys/net/ipv4/ip_forward contains a 1. If it's 0 then
echo "1" > /proc/sys/net/ipv4/ip_forward
I have used these rules successfully on Mandrake 8.0. Hopefully I didn't forget something.