Before you read this entire post – this was posted after I just installed the router and configured it for VPN (PPTP) pass through. A week or so later, when I was abroad and needed my VPN the most (of course), it had stopped working. To make a long story short – my router is now running the Tomato firmware. So unless you like banging your head in a buggy firmware from Linksys, read my post about installing Tomato instead.
You are using a Linksys WRT54GL broadband router to connect your LAN to the Internet. On your Internal LAN you have a computer that can handle inbound VPN connections (PPTP). This can be a Windows XP or Windows Server of some kind.
To make this work you must configure your broadband router to forward the inbound VPN (PPTP) connections to your PC. This is normally simple but some people (including me) ran into some problems trying to do this on a Linksys WRT54GL router.