1 IP, multiple pc to access

hi all! i would like to know what is the easiest way to use Remote Desktop Connection using 1 IP, pointing to multiple pc's?

here's the scenario:

we're using Zyxel USG 200 firewall and have 1 IP address provided by our ISP. i have a couple of pc's that uses Windows XP because the software is too old that it will only run using XP. one of the staff wants to access PC #1 from home, and another staff wants to access PC #2. do i need to configure the firewall?

please advice...

thanks in advance!