This one ought to be a little more of a challenge than your normal "compute these numbers" or "print this sequence of text" puzzles:
Write a program that launches another program and when that other program tries to access a certain file the program causes it to open a different file.
The program launches Notepad.
Notepad attempts to open "orange.txt"
The program detects it and makes notepad open "green.txt" instead.
WinAPI is allowed, but your code will stand out better if you can get this some other way.
You can use any language, but it must be able to run on either Windows or Linux