How can i share a PPTP Connection to my local network computers using RRAS and Windows Server 2008 ?
Here is the scheme :
Local Network Computers ==> Router (Win2k8) (VPN Connection) ==> Internet ==> PPTP VPN Server
Is the connection set up in the RRAS as a VPN Connection, or is the VPN connection set up under the "Create a Connection" wizard?
If it's a VPN in the RRAS, then you can tell the server which network(s) are on the other side of that VPN connection, and assuming you have Routing enabled in the RRAS, it will automatically route all the connections over the VPN as required.
This will only work if your subnet on your local network is different to the remote network (e.g. if your network is 192.168.0.x and their network is also 192.168.0.x then it won't work, but if yours is 192.168.0.x and theirs is 172.0.0.x then it will be fine).
Unfortunately I don't have the time to do a how-to, but hopefully this will put you on the right track.
-- Update --
After seeing your comment on the question, I recommend going with what Michael Graff said, and get RRAS to do NAT on the VPN tunnel. This is the easiest way if you're not looking to split your network.