Simply put, for your intranet you will need to have "Integrated Windows Authentication" selected only on the "Directory Security" tab. Your users will login to the network and be authenticated for Intranet access (those without authenticating will have a popup box for credentials on their first visit to the Intranet). Next put this in you code where you want it.
Welcome, <%Response.Write (Request.ServerVariables("auth_user"))%>
That's it. Now if you want to get more technical with it you can add other fields to manipulate it.
You can make it bold:
Welcome, <b><%Response.Write (Request.ServerVariables("auth_user"))%></b>
Strip out the domain name if applicable (DOMAIN\username). The "8" below would be counted from the left to remove 7 letters of the domain name plus 1 for the \
Welcome, <b><%Response.Write Mid(Request.ServerVariables("auth_user"),8))%></b>
Convert the case
Welcome, <b><%Response.Write ucase(Mid(Request.ServerVariables("auth_user"),8))%></b>
Whatever you want, just mod it to your liking. You asked for "can and how". How about "yes and like this". I hope this helps.
drew