If something hijacks your IE's View Source command, here's what to do

Some applications, for example text editors can change the default View Source editor in Internet Explorer. How I hate when this happens, especially when the installation program doesn't ask if you would like this to be done.

Okay, here's the registry key you need to edit (or delete) if you want to return back to the default editor, i.e. Notepad.exe.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\
...View Source Editor\Editor Name

Of course, if you want to go the other way around, just create this registry key and restart IE.