Try this:
Option Explicit
Const FILE = "C:\bootmgr"
Dim oFso : Set oFso = CreateObject("Scripting.FileSystemObject")
WScript.Echo "File '" & FILE & "' exists? " & oFso.FileExists(FILE)
When saved to a file (say noext.vbs) and run on my Windows 7 machine, I get:
File 'C:\bootmgr' exists? True
I'd think you'd get a similar result within your ASP Classic script.