Microsoft Excel Help, how to make a cell unchangable once any value has been displayed in it?

In Microsoft Excel 2003 how can I make a cell unchangeable by a formula once any value has been shown by a formula in that cell?

Microsoft Excel Help, how to make a cell unchangable once any value has been displayed in it?windows

By protecting your worksheet. Select that cells that you will allow changes to be made to. Then go to Format Cells and click on the protection tab. Click on unlock. Exit the dialog box. Now go to Tools %26gt; Protect Sheet. Anything that is not "unlocked" cannot be changed. To keep anyone from unprotecting you sheet, secure it with a password. Make sure the password is available because once you set this up, you can't undo it unless you know the password.