On 20-May-2007, ?B?RlN0MQ==?= wrote:

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)

I found a narrow RH column A1 downwards, conditionally formatted
with a formula to change the cells colour, give me a column where gaps
show unentered data; that is uncompleted tasks.
=(A30) is the formula, and I set color to red for this condition.
A3 being assigned a value is a completed task.
I then copied and extended the format downwards through A1 to Ann.
It's much easier to spot gaps in a black, red, or other colored line.
Might be useful.