Response.AddHeader "Location","http://www." & Request.ServerVariables("HTTP_HOST") & Request.ServerVariables("REQUEST_URI")

şeklinde tek satır yaparsan çalışır

If InStr(Request.ServerVariables("SERVER_NAME"),"www") = 0  Then
Response.Status="301 Moved Permanently"
Response.AddHeader "Location","http://www." & Request.ServerVariables("HTTP_HOST") & Request.ServerVariables("REQUEST_URI")
Response.End
End if

olarak çalışır.

Şöyle düzeltmek gerekir.

<%
If InStr(Request.ServerVariables("SERVER_NAME"),"www" ) = 0 Then
Response.Status="301 Moved Permanently"
Response.AddHeader "Location","http://www." & Request.ServerVariables("HTTP_HOST") & Request.ServerVariables("REQUEST_URI")
Response.End
End if
%>