On April 4, 2007 Microsoft issued a security update for Windows XP  "KB925902", that can cause editNC to fail. 
The update affects html-based help routines that are provided by Microsoft and used by the editor and many other programs.

There are two different ways to correct this error.
 
1) Remove security update "KB925902".

To remove the update, go to Control Panel> Add Remove Programs. Select the box at the top
that says “Show updates”. Search for “KB925902” and remove it.

Note: If Windows is set to automatically download and install updates, this one will
continue to be installed until Microsoft issues a fix.

2) Install Microsoft KB935448 update.  This fixes the error without a reboot.

The KB935448 update can be found here:

http://support.microsoft.com/kb/935448/en-us
 
(The update refers to the "Realtek HD Audio Control Panel" which evidently experienced the same problem.)