I already know that the standard tool that comes with Excel only works with
data entry cells, but because of the context of more complex spreadsheets,
there has to be a way to perform simple validations (which render a similar
dialog to the user) when the results of his/her work are invalid in
calculated cells?!

My task is simple: Cell C1 renders a simple sum Fx from cells A1 and B1.
Cell C2 = A2 + B2, etc. All I need to have Excel do is tell the user right
away whenever C1=C2, or in batches in the event of Copy and Paste.

Any help would be greatly appreciated!

Greg