Don't know if this is the best way, but you can just loop through the paths
in the PATH environment variable and look for the file in each one.  Here's
a console app that does it:

Imports System.Environment
Imports System.IO

Module Module1

   Sub Main()
       Dim fileToFind As String = "Notepad.exe"
       For Each pathName As String In
GetEnvironmentVariable("path").Split(";")
           Dim fullPath As String = pathName & "\" & fileToFind
           If File.Exists(fullPath) Then
               Console.WriteLine(fullPath)
           End If
       Next

   End Sub

End Module

- Scott Swigart