SRM,
Locking cells is a two step process.
Go to FORMAT>CELLS and then go to the Protection tab and for each range of cells you want protected or locked make sure the "Locked" option is checked. Conversely, for every group of cells you want to be able to change make sure the "Locked" option is unchecked.

Now, none of this locks or unlocks anything until you go to TOOLS>PROTECTION>PROTECT SHEET. After you protect the sheet all cells which have the "Locked" option checked will be unalterable and those that have the "Locked" option unchecked will be alterable.

There is a good deal more to learn about the many options with locking and unlocking and I would direct you to the Excel help file.



Hope this helps