i tried your same script(without changing single character) in my PC , its works fine for me. exactly its clicking file menu of notepad. if u r having problem then try ,instead of using "Window("Notepad").Click" try with "Window("Notepad").Activate",if u face same difficulty let me know which QTP version you are using?.
Try below code.
<font class="small">Code:</font><hr /><pre> Set fso=CreateObject("WScript.Shell")
fso.AppActivate("abc.txt") ' Mentioned name of your notepad file
I think you can use earlier approach by defining the Notepad object in OR or using DP. But I have not tried due to problem in my QTP license.