Access your Mac from XP...

This is just a simple way to remote into your Mac from XP. I am sure there are some alternate techniques, but this is fairly simple.
Ok so here are the steps I took:

1. Download OSXvnc (Vine), mount, and then drag into Applications.
2. Open System Preferences > Sharing > then click Remote Management (select the options you want).
3. Now launch OSXvnc Server and enter in the password to connect.
4. Log into your router and find the Port Forwarding tab. For Linksys routers it is under Applications and Gaming.
5. Now forward port 5900 (TCP and UDP) to your computers IP address.
6. Go back to your Mac and open up a browser and find out your external IP.
7. Go to the machine running XP and download and install RealVNC.
8. Open Up RealVNC player and enter in your external IP.
9. Enter in your password to connect.
10. Now that's it!

Now some explanation of what is going on. You have a service on your mac that is listening on port 5900. You are telling your router to forward all activity on port 5900 to your Mac IP. So, by entering your external IP, the packets are hitting your router, then forwarding to your Mac on which the VNC server is listening on port 5900.