It's me again, Keith. Do you want this to execute in every worksheet of workbook, or only in a particular worksheet? I ask because for both solutions you'll write approximately the same program, but if it's to operate on only a particular worksheet then you'll store it in the "Excel Object" that corresponds to that worksheet, but if in every worksheet then you'll put it in the ThisWorkbook object. You can see those in your VBA editor under "Microsoft Excel Objects" rather than "Modules".

(And by the way I'm assuming that you already know something about the VBA editor. If you don't—if this'll be your first macro—I should back up and start at the beginning.)

The macro you write will be named "Worksheet_Change"—it must have that exact name, because that way you cause it to respond to an event with that exact name—and it has to have the prescribed list of arguments with exactly the right data types, though each argument can be named what you choose. All this is defined here; take a look. Within those limitations your macro can do whatever you like.

I have a short Worksheet_Change macro that looks like this:This goes in a timesheet I maintain. Whenever I enter a value in one of the columns headed "From", it put's today's date in the date column. If it already had today's date in that cell, fine; but if it had some other date (in case I made a mistake) it displays a warning message. That's all it does. Maybe you can use it as a sort of template.