3 PC home LAN remote access problem

Set-up : PC#1 1 vista (cabled), PC #2 1 XP pro (Wi-Fi), PC #3 1 laptop XP
pro (Wi-Fi) Linksys router WRT54GS

From customer offices, we need to access via RDP our PCs, i.e. physically
access our desktop.

For some reason, I can access one PC at a time via RDP. Assigning different
RDP port to each Static IP and enabling all doesn't work. Those other ports
are added to PCs firewalls.

Is there another solution to access via RDP to choose which PC we want to
connect to ?

I also tought of using the VISTA machine as the entry PC in which I would
install something else (software I guess) to access the other LAN PCs, VISTA
serving as a tunnel and that would be beneficial since at home office, we
would be able to RDP locally other PCs.

Thanks