Since Active Server Pages (ASP) pages are compiled and executed before being sent to the client (as static HTML), trying to use a variable in place of a file name in a Server Side Include () fails. To include a file in your ASP page, you could include code similar to the following that reads in a file and displays it.
<% ' The name of the file to display was passed by a form using GET method
infil = Request.QueryString("file")
If infil <&>"" then
set fso = Server.CreateObject("Scripting.FileSystemObject")
set fil = fso.OpenTextFile(infil)
outstring = fil.ReadAll
' PRE tags preserve the format of the file
Response.write "<PRE>" & outstring & "</PRE><BR>"