||I finally found a thread from 2004 that gives the correct answer. A two+ year delayed "thank you" to Dan.
Subject Re: ArcMap - Field Calculator "User interrupt error"
Author dan rosenbalm
Date Jun 08, 2004
Message We have found a few solutions for this:
Workaround - Comment out all dim statements in the calulation script (Use a ' character at the beginning of the line).
Fix - Delete the arcid module from normal.mxt. To do this, open the VB editor, and expand normal.mxt in the project explorer window. Select ArcID in the module folder and delete it.
We had a bad reference in the ArcID module of the normal.mxt. Your ArcMap may be hanging up on an object in there as well. To check if you have the ArcID module, open the VB editor, and expand the normal.mxt tree in the project view. The ArcID module (if you have it), is in the modules folder.
Once you delete the problem entry, or the entire ArcID module, your scripts will work fine again.
Make sure you don't need the ArcID module for other customizations to run before you delete it!
The ArcID module can be unintentionally created by pushing the update ArcID module button on the options tab of the Customize menu.