There is a lot to this (more than you probably imagined at first).
1. Considering you most likely have a dynamic IP, you need to register with a dynamic DNS service, such as dyndns.org or no-ip.org. You will need to install an agent on the Dell so it will keep the hostname updated with the correct IP, in case your IP changes at some point.
2. Set your Dell to a static IP on the internal network and enable remote desktop (Right-click My Computer -> Properties -> Remote tab -> Enable second checkbox (allow remote users...).
3. Then you need to configure your router to allow TCP port 3389 through it and to the new static IP of your Dell.
4. Now get the Terminal Services client from Microsoft and connect to the hostname you configured in your dynamic DNS service (step 1).
Note that when you connect via Remote Desktop to WinXP, the console locks. This means only one person can use that Dell at any given time.