you might also consider using Conditional Formatting - ie let the cell turn
red or something to make it really visible

"Stuart M" <Stuart wrote in message
...
Cell validation only works when data is entered directly in a cell. I want
to
recreate this effect so that Excel displays a pop up message when the
calculated value of a cell falls below £100. The cell in question is
calculating the sum of three other cells on the worksheet and is formatted
as
currency.

Cheers :)