DDNS is your option. I have been using dyndns.org and it works OK.

DDNSWhat's DDNS ERROR: Remote Security Gateway domain name problem I have an account with dyndns but get "block for abuse" My DDNS log lists error "nohost" ...
     www.chicagotech.net/ddns.htm  

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net 
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com 

  I am trying to establish a VPN connection to a remote network. The computer
 that will be receiving the incoming VPN call is a tower running windows XP
 Pro. When I am within the network I can make the VPN connection, using the IP
 address of the tower. But when I try a remote access, using the WAN Ip
 address of the router I get no response. I have configured this router to
 redirect incoming calls to the Tower's IP address and I have temporarily
 disabled the Tower's firewall to rule that out. What I haven't done is get a
 static Ip address, apparently comcast doesn't offer them to residential
 customers. Can this be done with a dynamic address? If anyone has any
 suggestions I would appreciate it.

 Thank you,

 Dan

 The model router is: D-link DI-604
 Virtual server settings: PPTP TCP 1723/1723