Run REGEDIT, follow the following directions to the proper key.

HKEY_LOCAL_MACHINE
|- Software
|-- Microsoft
|--- Internet Explorer
|---- View Source Editor
|----- Editor Name (Default) = C:\windows\notepad.exe

If this doesnt exist (but it should) then create the Key "View Source Editor".
Then create a Key within that named "Editor Name". Modify the "(Default)" value to make it point towards any program on your computer using "D:\Tools\Notepad2\Notepad2.exe" (without the quotes).