This simple but important piece of code checks if a file exists . If it does a message is displayed , if it does not exist a different message is displayed.

<%
Dim objFSO
Set objFSO = Server.CreateObject("Scripting.FileSystemObject")
If objFSO.FileExists ("yourfile.txt") Then
Response.Write "File exists"
Else
Response.Write "File does not exist"
End If
%>