PPTP registry entries are at:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RASPPTPE\Parameters\Configuration

AuthenticateIncomingCalls is a type REG_DWORD which defaults to 0. A 1 forces the PPTP protocol to only accept calls from IP addresses listed in the PeerClientIPAddresses value.

PeerClientIPAddresses is a type REG_MULTI_SZ and contains a list of IP addresses, one per line.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<adapter name>\Parameters\Tcpip

DontAddDefaultGateway is a type REG_DWORD which defaults to 1. An entry of 0 will disable a default route for your LAN adapter.

PPTPFiltering is a type REG_DWORD which defaults to 0. A value entry of 1 causes the adapter to only accept PPTP connections.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

PPTPTcpMaxDataRetransmissions is a type REG_DWORD which defaults to 9. It represents the number of times that a PPTP packet will be retransmitted if it is not acknowledged. The value is set higher than the default TCPMaxDataRetransmissions value to prevent dead gateway detection from occurring on congested Internet links.