If you turn off anonymous acces to your site (or just that page) then the browser will send the user credentials to the server without the user having to do anything. You can get the domain\username through Request.ServerVariables("LOGON_USER"). The client's IP address is available in Request.ServerVariables("REMOTE_ADDR").
You don't say exactly why you need this info. Do you just want to restrict access to certain users? If so, one way would be to create a new NT Group of the users allowed access. You can use the domain\username to to check if the user is a member of that NT Group via ADSI (see
http://www.15seconds.com/issue/020130.htm for examples). You should try to avoid having a separate user db for you site if possible. In an intranet scenario you should be able to handle things like this using just windows.
hth
Phil