See solution in other versions of Excel:
Question: In Microsoft Excel 2003/XP/2000/97, I want to set up a cell to only allow positive numbers. How can I do this?
Answer: Select the cells that you wish to restrict to a certain number of characters. In this example, we've selected cells A2 to A6.
Under the Data menu, select Validation.
When the Data Validation window appears, set up your criteria. In this example, we've setup the cells to allow a positive number.
Next, click on the Error Alert tab.
Enter an Error message to appear when data entered does not conform to the validation rules.
Now if a value is entered in one of those cells is not a positive number, the following error message will appear: