Basically no, you can't change it. The TCP port is only used to setup the initial connection. All traffic is sent over GRE, not TCP. I highly recommend requiring client certificates. PPTP as a protocol is plenty secure when you pair it with client certs, no need to upgrade to something like SSL VPN.
See this question from the other day for links on how to setup this up.
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
Use different username in PPTP client configuration in each computer to connect to pptpd server. And then in
/etc/ppp/chap-secretsyou can map username to IP address as following.The fourth column assigns static IP address to the user in column 1. Now though you cannot use hostname, at least you have fix IP address for each computer. So you do not have to check IP address of the computers manually.
OR if you have local DNS server available,
Then you add
Arecords for each computer in your local DNS server to point hostname to the static IP address you have assigned for the computer inchap-secretsfile.