It's funny I stumbled across this because I just changed IE this morning to do just that, but I found instructions for editing the registry at
http://www2.chemie.uni-erlangen.de/peop ... tails.html
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(requires I.E. 5.0 or later, you have to have WRITE access to registry on
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer key)
Click on Start, select Run..., enter regedit, and press Enter.
Navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer.
If a View Source Editor key (item with folder icon next to it) does not exist, right click on the Internet Explorer key and select Key from the New menu to create one. Its name must be exactly View Source Editor.
Again using the Key on the New menu of the right click menu, create an Editor Name key inside the View Source Editor key if one does not exist.
When you finish, the fully qualified path to where you are currently in the registry (displayed in the status bar at the bottom of the screen) should be HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\View Source Editor\Editor Name
In the right hand pane double click on the listing for (default). Dialog should pop-up prompting to edit value. In the resulting string box enter a fully qualified path to a program file in the form of x:\directory\filename.ext surrounded by double quotes, i.e. "D:\Programs\some_editor.exe" and then click the OK button.
Internet Explorer must be restarted in order to changes will be taken into account.