Probably a file type or registry entry has gotten fouled up.
Bill James MVP has a script file available that will solve this problem
easily.
http://billsway.com/vbspage/
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).
Or go to My Computer...Tools...Folder Options...File Types tab. Scroll down
to the .htm file type, click to highlight, click Advanced, under Actions
click on Edit to highlight, click Edit button.
Action: &Edit
Application used.. : "C:\Program Files\Microsoft
Office\Office10\msohtmed.exe" %1 [for FP in Office XP]
Use DDE checked
DDE Message - [blank]
Application: msohtmed
DDE Application not running - [blank]
Topic: System
Okay out and do the same for the .html file type
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/?kbid=163606