I'm assuming you are wanting to be able to connect to your computer over the internet, and not just WIFI. If I'm correct, what you must do is forward some ports.
First you will want to make sure remote desktop is ready to be used on your computer, I also use XP so I will try to give a step by step instruction.
1. Click on the Start Menu then right click on "My computer" and go to the "Remote" Tab. Make sure both "Allow Remote Assistance invitations to be sent from this computer" and "Allow users to connect remotely to this computer" are checked. Then click apply and and ok.
2. Now you are going to want to find the local IP Address of your computer, to do this go into the start menu and click "Run" then type "cmd.exe" (without the quotes) and hit enter. Then type "Ipconfig" (again without the quotes) into the cmd command line, and there should be a few things that come up look for something that says "IP Address . . . . . xx.x.x.x".
3. Now you are going to have to forward ports on your router, I am assuming you know how to forward ports on your router (If you don't
this link should help). You are going to want to forward port 3389 to your IP Address found in step two.
4. You need to find the IP Address of your router you can do this by going to a site like:
this.
5. Now you are going to go on your phone and start the Remote Desktop application. In the "Computer" box you will type in the IP Adress found in step 4, in the "User Name" box you type in the user name you want to log in as, and in the "Password" box you will type in the password of the user name.
If you have any questions about this let me know and I will try to help.
Edit:
If you want to connect to your computer over WiFi, just skip step 4 but do all the rest, and for step 5 instead of typing the IP Address found in step 4 you will use the IP Address found in step 2.