VPN Help

I wonder if anyone else has come across a problem I’m having. I have a Linksys WRT54G Router and running Windows XP. I have been trying to set my pc up as a VPN server without any luck at all, my router supports vpn passthrough and I have enabled support in the xp firewall, but I have yet to find ANYONE that can connect to me using VPN. I can however, connect to someone elses machine through a vpn without any problems. I can also establish a vpn within my network on any of the pcs. I have tried everything, my pc is in the DMZ, I have opened specific ports on the router, and still nothing……..HELP!!!