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 \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"