If your trying to use the "Remote Desktop" Feature that is built into Windows XP and are Hosting your Remote Desktop PC from behind a router, you'll most likely need to open up a port in your router for forwarding. Otherwise you wont be able to remotely access your hosted computer from another PC via the internet. The following tutorial assumes that you already know how to navigate and setup port forwarding in your router and that you simply need to find out which port to forward for Remote Access. Navigating particular router software is outside of the scope of this article.
Question: What is the port used and necessary to be forwarded for Remote Desktop access?:
Answer: Port 3389 is the default port used for (Remote Desktop Protocol). This is the port you would forward in your routers port forwarding section. An example screenshot is available below for port forwarding using a Linksys Router (Applications and Gaming Section). Your Router and the software used for port forwarding may differ: