Request.ServerVariables("HTTP_REFERER") returns the URL that referred a visitor to your server.
If you use Response.Redirect, HTTP_REFERER will be empty.
You may want to consider employing a pre-made component for tracking the link usage / site statistics.
Here's a script that will allow you to track internal link usage.
CodeAvalanche LinkTracker: http://m.1asphost.com/xfairguy/lt.asp