When I attempt to connect to a server running XP x64 (so essentially Server 2003) using a PPTP connection, it fails with client-side error

Registering your computer on the network...

Error 720: A connection to the remote computer could not be established. You might need to change the network settings for this configuration.

and server-side error

Event ID: 20050

The user WINSERV3\Andy connected to port VPN8-1 has been disconnected because no network protocols were successfully negotiated.

I have configured the router to pass both TCP packets on 1723 and GRE packets. I have used Wireshark (filtering out ARP, UDP, and all TCP ports other than 1723) to observe the packets received by the server. Wireshark does not explicitly name any protocol GRE, but it does tell me the server sent and received TCP, PPTP, PPP LCP, PPP CHAP, PPP CBCP, and PPP IPCP. The connection seems to go wrong at packet 30, where the protocol is PPP LCP, with the payload of the packet being labeled "Protocol Reject". Obviously, this is going from server to client.

This would seem to lead to the conclusion that there is something wrong with my client, which runs Windows 7 Ultimate x64. However, it is able to connect to my house's router, which runs the DD-WRT firmware and is thus a PPTP endpoint. I'm thoroughly at a loss. Please help!