I know you don't want a registry hack, but I had to reverse a virus that did something similar, just about bashed my head till I figured it out.
Go to the HKCR\exefile\shell\open\commandHKCR\exefile\shell\open\command
key and change the (default) and IsolatedCommandIsolatedCommand
keys to notepad.exenotepad.exe
Then everytimeevery time they go to execute a program they will get a copy of notepad instead.
To fix, you can still run the programsasprograms as long as you do it as another user (unless you change those keys as well. :) )