User authentication on Web Service calls from an NTD app

[Miceal]

I am developing a Windows Forms application that makes
requests to a web service and in return receives
configuration data that is user specific, i.e. based on
the user's window's authenication on the server side
domain. My problem is that I am unaware as to how I
should configure the app so that when it calls into the
web service, the web server will be able to identify the
user. Initially the application will be running on the
same domain either over a LAN or though a VPN connection
although it is anticipated that in the future connections
may be made from a m/c outside this remit although the
latter scenario does not have a high priority right now.
Any illumination on this topic will be gratefully
received. Thank you.