Data validation in excel

Question

How to use datavalidation in excel when the cell or particular range is calculated with the formulas. i think that data validation will happen only when any value is entered in particular cell but i need the error message when ever the value changes.

for eg,

A1 contains 0
B1 contains 1
c1 is calulated with the formula =A1+B1

when ever the value is change in A1 or B1 the C1 will be automatically changed the cursor may not go to the particular cell.I need the error as "This less than zero".