User Authentication from NT Accounts

I am trying to find a way to create authentication for our employee website that will use NT authentication instead of having to maintain a seperate database for the authentication information.

I did find some code at:
http://codewalkers.com/seecode/111.html

But I do not really understand what it is doing or how to go about setting this up to even test it. Does anyone know anything about doing this or understand what the code is doing that is listed on that page? Thanks!