Perhaps set up a user form to initiate the process but leave it showing?

Interesting idea.
I do this all the time with VB6, but I can't seem to get this
to work  in Excel.

I have a command button on Sheet1 named cbSTART.
I created a form and changed these properties manually:
-- (Name) -> FormTest
-- Caption -> FormTest
-- BackColor -> Light green

How do I get the form to appear? This is what I tried...

Sub cbSTART_Click()
-- FormTest.Top=500
-- FormTest.Visible = True <<---------- crashes here
End Sub

"How do I get the form to appear?" <== if i understood this question right then
i can prefer this:

Sub cbSTART_Click()
Formtest.Show
End Sub

The good news is that .Show worked ! Thanks
The bad news is that once the form is ".shown", the
Application.OnTime function  suspends (until the form is
closed). Thus, this is a dead-end.

Thnaks anyway.

And what about the other idea? I mean forcing the user only choose Range("A1") ?

I have experienced the same problem. I'm using the
the following type of syntax

Sub MainTimer()
-- TmrTime = Now + TimeValue("00:00:01")
-- Application.OnTime TmrTime, "Sheet1.MainTimer", EndTime
-- >> other code <<
End Sub

The sub constantly calls itself (once per second) until
EndTime is reached (defined elsewhere). If I begin typing
something in any cell on Sheet1, the MainTimer stops
working until I press the <enter> key. I don't have
a solution to prevent this from happening.

In the VB6 world, where everything is form-based, I've
used something like this:

Sub Form_KeyPress(KeyAscii As Integer)
-- If isBLP Then
---- Exit Sub
-- End If
-- >> other code <<
End Sub

If the variable isBLP evaluates to True, the sub is exited.
Otherwise, the keyboard  entries are processed. I'm afraid
that I don't know of a parallel in Excel when you are
"sheet-based" or "application-based".

Anyone else have any ideas?

At the beginning of your routine  enter

Application.EnableCancelKey = xlDisabled

One warning, if there is an error in a loop you want be able to stop the
program.

I assume that you meant "won't", and not "want" (hehe).

Although that wasn't what I was after, it gave me an
idea -- use Application.OnKey

The brute force method would be something along the lines
of...

Application.OnKey "a", ""
Application.OnKey "b", ""
Application.OnKey "c", ""
Application.OnKey "d", ""
.. etc.

This disables each of the keys "a" to "d", one at a time
(note, it is case sensitive, so "A" is still live)

A more elegant way would be:

For ii = 33 to 128
-- Application.OnKey Chr(ii), ""
Next ii

The above will disable any printable character. You could
possibly expand the scope of the loop to handle others.
However, navigation keys and such have specific references
(check out "onkey" on VBA help), although I doubt that they
are giving a6 any problems.

It seems that this loop can go in any sub that gets executed
before the timer  sub begins.

As I read through all the good suggestions, it occurred to me that if
I could just stop any selection of a cell on any of the worksheets
with a mouse click -- i.e., not be able to select or focus on a cell
or cells, that should take care of the problem. Is that possible?

Good idea. Then i prefer this;

Choose a cell which has not important data. I assume "A1" is this one.

Then put this in the program:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
ActiveSheet.Range("A1").Select
End Sub

That means when the user choose a Range or Ranges it will be forced to choose
onle the Cell "A1". He can just change the data  in "A1" .

Is it close to what you're looking for?

Does this stop only the Cancel Key or any key/mouseclick that focuses
on a cell in the workbook?

I have found that selecting a cell on any of the worksheets  while the
timer or routine  is running  may or may not interrupts the timer. I can
select another sheet and have no problem, I can scroll slowly through
a sheet with no problem. A mouse click in any cell, even an empty one,
will sometimes interrupt the timer; accidently entering something from
the keyboard  into a cell always interrupts the timer. Since I want to
work elsewhere on the desktop while the program is running, I am
looking for a way to make  the sheets impervious to any external entry.

I know this is not exactly what you want but very simple:

ActiveWorkbook.ActiveSheet.Visible = xlSheetHidden

Now the sheet is invisible. Son no Entry from keyboard  or mouse can reach on
it.

Good idea, but I need to see the results of my processing  as they are
posted to the sheets. So, I want to sheets to be impervious to
external interruption, not invisible.

If i didn't understand wrong then i think this helps;

During the process just activate a little userform which has label like
"Please wait during the Calculation Process" and leave Userform's ShowModal
property "true". As you know, that means the sheet is visible but unreachable
during the Userform is active.

I think somebody already gave this advise,didn't he?

Anyway, i like simple solutions and hope this simple one works