Note: This information is for network administrators and those who understand how to configure computer systems that have firewalls or network address translators (NATs).
If you have trouble using voice and video communications, application sharing, Whiteboard, or Remote Assistance in MSN Messenger, it might be because you or the person with whom you are communicating are behind a firewall or network address translator (NAT) that prevents a direct connection to the Internet. Because your Internet service provider might be using a firewall or NAT, you might be prevented from connecting, even if neither of you are behind a firewall or NAT.
To allow voice and video communications and the use of application sharing, Remote Assistance, and Whiteboard from behind a NAT:
Ensure that your NAT is Universal Plug and Play (UPnP)-enabled. If you are not sure what type of NAT you have, check with your computer manufacturer or NAT vendor.
Configure your NAT so that all MSN Messenger features work. For more information, see Windows Messenger in Windows XP: Working with Firewalls and Network Address Translation Devices.
http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/winxppro/deploy/worki01.asp