By saying ASP.NET i assume you meant you like to get the user id of logged in person on client machine accessing your intranet web application.
Try this using JavaScript
var wshshell=new ActiveXObject("wscript.shell");
var username=wshshell.ExpandEnvironmentStrings("%usern ame%");
or this
var ActX=new ActiveXObject("wscript.network");
alert(ActX.username);