I've tried to set up the pptp connection in ubuntu with the default settings, the connection works in windows 7 also in windows xp, it even works with ubuntu within same provider's network. I've tried to connect with two different computers on my home network (both pc are able to connect with vpn in windows 7) but with ubuntu i got error vpn connection failed because vpn stopped. I found this VPN connection failed because service stopped unexpectedly but that did not help me. When i tried pc within same provider's network is working with the default settings (so only the gateway, name and password are set), I've also checked router's setup and there is no problem, it is possible that my isp blocks the linux vpn, but not in windows?
For setting up a pptp vpn on lubuntu I went through a lot of trouble.
The details on how I managed to do so are described in this link.
Maybe it can help you.
In searching for a solution, I read a lot of warnings that pptp is unsafe and should be avoided where possible, so I carefully pass this warning on.