Excel cell protection works on two levels.
Individual cells are Locked by default - go to the Format menu, choose Format Cells and click on the Protection tab. Here, you can change the "Locked" status of the currently selected cell(s).
However, this Locked status is ignored unless the Sheet as a whole is protected - go to the Tools menu, select Protection and Protect Sheet. Once you do this then the Cell protection comes into play.
The process is to first use Cell Protection to unlock all cells that you want users to be able to modify, and then protect the sheet.
This applied to Excel 2008, but from memory it worked exactly the same in 2004.
