Configure Windows XP Pro to be a VPN server
Not 100% certain, but I'm pretty sure that's what would need to be done.
And as a little footnote: MS Terminal Services/Remote Desktop (comes with XP Pro) is way cooler than VNC is, especially if you install the SP2beta patch that enables concurrent sessions. You can be using your computer remotely in one account, and have the console session still available at your home computer if someone were to walk up to it. And other cool stuff like clipboard sharing, resolution scaling, file transfer, audio redirection... it's just better.