 <p>You want to check out <a href="http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/523ae943-5e6a-4200-9103-9808baa00157.mspx?mfr=true" rel="nofollow noreferrer">Integrated Windows Authentication</a>. This will allow the Active Directory username and password (hashed) to be sent across the network to the server. If they pass you can redirect them to the site, and if not, push them to the login page.</p>