The default application that Internet Explorer uses for showing source code is NotePad (meh!!!).
If you want to use a different application (I suggest NotePad++), then add the following key to the registy:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\View Source Editor\Editor Name] @="\"C:\\Program Files\\Notepad++\\notepad++.exe\""
You can copy the above code into a text file, rename it to a .reg extension and double click it – to automatically incorporate the settings into IE. (Change the exe path to your favorite html editor).