No UPnP detection when using MSN Messenger behind WinXP ICS

I have two computers with WinXP-SP2 connected to an internal network, both
are getting their IPs from another DHCP server.

Computer A is dialing to the internet through a VPN interface, and shares it
to the network with ICS.
Its firewall is configured to enable UPnP framework.

Computer B gateway and DNS server are manually configured to be computer A.
In other words, computer B is connected to the internet through B.

When connecting with the MSN Messenger to the internet, the information I
get under Tools-Options-Connection, on the "Advanced Connection
Information" is that I'm connected through a non-UPnP IP restricted NAT.

The way I understand it, after reading a bit on microsoft documentations, I
should be connected throught UPnP, so what am I doing wrong?

Thanks for your help, AP