Hi,
try this (Excel 2003):
(sorry for any incorrect translation, I am using a Swedish Excel version).

Mark the cells you want to use (for example A1:G20)
Select menu: Format / Cells. Click the tab "Protection". Unclick the checkbox "Locked".

To make this choice active (the "Locked" you did above), you must now protect the whole sheet.
Select menu: Tools / Protection / Protect sheet.
Before pressing "OK", unclick the checkbox "Mark locked cells".

Kind regards
Bjorn