HKCU\Software\Microsoft\Internet Explorer\Default HTML Editor
Change 'Description' to Notepad++
Goto ...\Shell\Edit and change 'Command' to the program path (C:\Program Files\Notepad++\Notepad++.exe %1).
An alternate way to associate Notepad++ with editing html files without the need to edit the registry:
1. In an explorer window, select tools-> folder options, "File Types" tab.
2. Select your file type (Firefox document for me, batch file for you) and hit the Advanced button.
3. Create a "New" action, call it "edit", and give it an action of '"C:\Program Files\Notepad++\notepad++.exe" "%1"'
4. Make sure "Open" is the default!
Yeah, NPP needs to do this on its own :-p
Hmm, right no doubt it will work or I can get it to work (ie7)
In general, However, installing N++ ought to figure this out and extend that little drop-down list that ie7 provides on the Internet options programs tab properties.
Maybe it could interactively heavily suggest that the default editor be changed to N++ unless you leave the default as it was before.
OpenOffice manages to add itself to this list - well apparently, as I haven't actually installed N++ or OOff AFTER having updated a system with ie7 so I could be misleading you there.
p.s. If M$oft made it easier to extend/change the list that would be nice too!