In IE go to Tools...Internet Options...Programs tab and select one of the other two in the dropdown list besides HTML Editor, click Apply, click Okay.
If the program you want to use is not among the choices there and you'd like to add it, read on....
Bill James MVP has a script file available that will solve this problem easily.
http://billsway.com/vbspage/ - Add new Internet Explorer Editor.
You will need to know the path to the editing program you want to use.
One small reminder... Bill's utility is quite nice but it will not populate
the DDE entries for the new file action, if they are required by the application you choose.
With most basic uses, there should be no issue - DDE will probably not be
used. 'Multitasking' users (many docs /applications open at once) may want
to put those DDE values in place and as far as I know that must be done
manually. (clone the winword.exe entries for the *.DOC file type if you are using Word).
You could also do the registry editing manually, if you wish:
http://msdn.microsoft.com/workshop/browser/configuration/ClientReg/ClientRegistryLayout.asp
Also see this article for more information:
Changing Source Code Editor in Internet Explorer
http://support.microsoft.com/support/kb/articles/Q163/6/06.asp