Another great free tool that I have found and use on a regular basis is found at http://www.logmein.com
They offer a free service that allows you to remotely access your PC securely from anywhere on the internet. While Windows XP Pro has remote desktop, one of the really great things about this tool is it allows you to set up multiple pcs behind a router without having to open a port on your firewall for everyone. It works on XP Pro, as well as home and windows 2000. I also use it to access pcs that are on networks for monitoring the network itself. The best thing about it is that it is a free product. No gimmicks.
If you need to access your pc or multiple pcs on your network I would suggest downloading this product and give it a try.