To get Internet Explorer to use your HTML editor when you press View Source, modify the following .reg file. Double-click it or run it (regedit /s <Path>\filename.reg):
Windows Registry Editor Version 5.00            (or REGEDIT4)                              \[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\]                              "Description"="Notepad"                              \[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell\]                              \[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell\edit\]                              @="&Edit"                              \[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell\edit\command\]                              @="C:\\WINNT\\notepad.exe %1"