For windows it's a pain to change the default port numbers that some services listen on. What you can do is on the virtual server page is assign different public port numbers for each PC and have it map to the default private port (the default port number that Remote Desktop uses) for each PC. For Example:
Default port number for RDP: 3389
PC-A 192.168.0.105
PC-B 192.168.0.106
PC-C 192.168.0.107
PC-A public port 4000, private port 3389
PC-B public port 4001, private port 3389
PC-C public port 4002, private port 3389
When you connect to a PC from the outside simply specify it in the remote desktop client in the format of [PUBLIC IP ADDRESS]:[public port number]
Another option
If you have an old PC laying around you can run Linux on it and use ssh like a vpn to access your home network. This option is more secure.