Android phone can't access anything through VPN on my Windows PC

I have a Galaxy S (Android 2.1 Update 1), and a Windows 7 Pro machine configured to allow incoming connections (i.e the built-in PPTP VPN functionality). I can establish a VPN connection to the laptop from the phone however I can't ping it or access a simple HTTP service running on 8081. Windows Firewall doesn't seem to be the issue as it doesn't work with it turned off either. It makes no difference if I use my house Wifi or disable that on the phone and connect via the phone's broadband connection. I notice via IPCONFIG on the laptop that the subnet for the phone's RAS connection is 255.255.255.255 whereas the network is 255.255.255.0 - could this be a problem?