Multiple Remote Desktop

I have a customer who has a need to allow 3-5 users to connect to their XP PCs.
I would like to have them use remote desktop to access their work PCs for home without using a VPN.
I have tried changing the listening port on the XP PCs so I could redirect traffic from the firewall to individual PCs based on the port but remote desktop does not work with port change.
Is there any way to use port translation rather that port redirection to accomplish the task?
(IE: taffic for port 3356 redirected to a particular internal IP on port 3389).
The enviornment is a 2k SBS, 15 XP pro workstaions and a Netgear fv318 firewall (not capable of port translation).

any sugestions?
JT