Locking and Unlocking Cells :
http://www.wellesley.edu/Computing/Office03/Excel03/excel03.html#locking_and_unlocking_cells
By default, Excel "protects" cells, graphics, charts, and other worksheet objects. This protection will only take effect by turning on the Worksheet Protection option in the Protect Sheet dialog box. Therefore, if you enable protection, no changes can be made to a cell until you unlock that cell.
Locking Cells:
You can easily lock any cell in a worksheet. After you Unprotect the Worksheet, complete the following instructions:
Select the cell(s) to be locked.
From the Format menu, select Cells.
The Format Cells dialog box appears.
Select the Protection tab.
Select Locked. (There will be a check before it.)
Click OK.
The cells are locked.
Unlocking Cells:
In order to unlock cells, sheet protection must first be turned off. See Worksheet Level Protection or Workbook Level Protection for more information.
Select the cells you want to unlock.
From the Format menu, select Cells.
The Format Cells dialog box appears.
Select the Protection tab.
Deselect Locked. (There will no longer be a check before it.)
Click OK.
The cells are unlocked.
