There is a nice registry hack that allows you to execute a specific application when you try to execute actually a different application. For example if you try to run notepad.exe, the calc.exe will be launched instead.
Details:
The key you create in HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Options must have the name of the application you want to take over. For example you can create a new key called notepad.exe and then create a new string with the name Debugger and value C:WINDOWSsystem32calc.exe.
Now if you try to run notepad, the calculator will be launched instead.