Remove any embedded spaces from the below link:
http://www.practicallynetworked.com/support/linksys_router_help_pg3.htm#PPTP
If you are using V1.30.5 or higher firmware, make sure you enable PPTP pass-through on the Filters Admin page.
PPTP is Microsoft's protocol for Virtual Private Networks (VPN). If you have PPTP Clients on your LAN, you just need to enable PPTP pass-thru as noted above. You also need to properly configure a VPN connection profile in each of your PPTP clients.
NOTE: Although multiple PPTP clients can be passed thru, only one PPTP client can connect per PPTP server. (This is a common limitation for inexpensive NAT routers.)
Many users have reported problems accessing a PPTP server on the LAN side of the router from the Internet. This problem was fixed as of the V1.23 firmware update, so just upgrade to the latest firmware if your firmware revision is lower than 1.23.
NOTE: No matter which version firmware you use, you will need to set the PPTP server computer as the DMZ computer and remove any Port 1723 forwarding.