I followed your suggestion to set a watch for a change and ran the following
code.

Sub test()
Application.ScreenUpdating = False
Range("a1") = "aaa:"
MsgBox (Application.ScreenUpdating)
Application.ScreenUpdating = True
End Sub

The results:
The messagebox said "False" as expected. The watch stopped at each
screenupdating call, to signal that a change had occurred; again as expected.

However, something is badly messed up. I stepped through the test code to
see what I could find. After executing "Application.ScreenUpdating = False",
I hovered my mouse over "Application.ScreenUpdating" and it said "True". Very
strange!?! After it executed the next line [Range("a1") = "aaa:"], it
immediately updated the screen. Now that's bad!?!

So now I'm wondering what is going on. Any help will be appreciated.