Assuming all the PC's are XP Pro then just use Remote Desktop. You can forward the ports through the SME Server to the PC's
i.e forward port 3390 -> PC A Port 3389
3391 -> PC B Port 3389
and so on.
You can then access your LAN PC's remotely with the RDP client by using the url domain.com:3390 for PC A, domain.com:3391 for PC B etc or you can use IP address xxx.xxx.xxx.xxx:3391.
To make it even more secure you can always set up SSH tunnels and RDP via the tunnels.
Jon