Hi Bumblybee,
Wow, this has far more possible scenarios to protect against than I ever imagined you were wanting. I felt that with customizing only a few things, you might be able to get by with it, but even with just messing around with just a few user settings and menu items, you're surely going to adversely affect the use of other workbooks. - Even when you try to have everything being restored using deactivation, and closing event's, you will never have everything switching back and forth between users original settings and yours, during the simultaneous use of workbooks, and/or running in different instances of Excel. And worse yet, if excel or windows crashes, (which it will sooner or later), it's nearly impossible to record and restore the original user preferences.
So sorry that I didn't say much about the downside to all this sooner, but I think you have no reasonable way to do what you want in Excel 2003
--- Take a look at using Excel 2007/2010, as the menu customizations are far more adaptable, robust (and straight forward, once you get the hang of it). And all the modifications can be done at the workbook level.
One of the first things to find out before moving in that direction is to make sure there is a stable way to go about making an .xlsm rendered non-functional when opened in 2003, even when the 2003/2007 compatibility tool is installed.
[]