Hi Lynne,
By default, all cells are set to locked. In the FORMAT menu, select CELLS.
In the Format dialog box that pops up, select the PROTECTION tab, and you
will see that the current selected cell is locked. This is so for all cells.
This means that when you protect the sheet (TOOLS > PROTECTION > Protect
Sheet) all the cells will be locked, and no-one can make any changes,
anywhere.
So, select all the cells you want other users to access, unlock them as
above, then protect the sheet.
Now if someone tries to over-write a formula, they'll receive a message
telling them they can't.
Hope this helps
Regards - Dave.