Go to

View-- Toolbars---Then go to Commands... scroll down and click on Forms

In the forms commands you should see the check box that you need.

" wrote:

It would be useful if I could have a box in Excel.

When I click on it I want it to turn from a square to square with an
'X' in the middle (or something similar)

(the purpose is so that I can quickly 'tick' tasks off when I have
done them)