Problems with ServerVariables

I currently am having a problem with my webserver. I am trying to get the Logon_User from the servervariables. i am using the following in a vbscript:
[code]
<%
strLogonName = Request.ServerVariables("LOGON_USER")
%>

<%=strLogonName%>
[code]

When I run this page I only get a blank page. No logon name shows up. Can someone please help me with this.

Thanks in advance,
Nate