All cells in a new worksheet have Protection property set
on (Format/Cells/Protection) by default, so that when you
protect the whole sheet, all the cells are protected. You
want to do the inverse. Try this...
Select the whole sheet by clicking on the upper left hand
square where the row and column headers meet. Go to
Format/Cells/Protection and uncheck the protection
option. Click OK. Now ALL cells are unprotected. Select
the cell you want to protect and turn it's protection
property back on. Now when you protect the sheet, all the
cells will be unprotected except the one you turned back
on.
Hope this helps.