The default program that shows the source code of webpage, when you are using Internet Explorer is Notepad.
If you want to change this defautl application, one way is edit the system registry.
I tested this solution with Windows Vista and IE 7, WinXp and IE6 and WinXp and IE7.

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

HKEY_LOCAL_MACHINE -- > Software --> Microsoft --> Internet Explorer

Create the key "View Source Editor" and then create the key "Editor Name".
Modif Default Value of the key "Editor Name" with the path of your Html editor and click Ok.

Restart Internet Explorer and try to choose View -> Source from menu.