It's not entirely clear to me how you got PPTP working to the Windows XP server when you have forwarded all port 1723 traffic to the Debian Squeeze server. You likely can only "VPN" to the WinXP server from within your local LAN, which seems to be of limited usefulness.
Regardless, PPTP requires not only TCP port 1723 traffic, but also GRE protocol. Is your router capable of handling GRE tunnels correctly? If it's an ordinary consumer-grade router, then I suspect not. And even if it is, GRE is esoteric enough that finding help may be difficult.
In your case I recommend trying a VPN solution that uses only TCP and/or UDP transports, since those protocols are ubiquitous and well-known. OpenVPN is one such VPN solution, and it is available for all the major OSes (Win, Mac, Linux, *BSD).
Depending on what you're trying to accomplish, another possibility is to run sshd on the Debian server, e.g.:
apt-get install openssh-server
All the major OSes have free ssh clients capable of creating tunnels over an ssh connection.
Best Answer
You know, I think you could probably do this using Windows Firewall with Advanced Security. When not connected to the VPN you should have a LAN connection that should be a Home network (private profile) or a Domain network (domain profile). When connected to the VPN you should have another network connection that is a Public network (public profile). Windows Firewall with Advanced Security allows you to create different rules based on the network type (profile) that you're connected to. You could create outbound rules that disallow the appropriate traffic for the Home or Work network (LAN) but allow it for the Public network (VPN).