Say you wanted to restrict this to Sheet 1 Range A1:C20, then the
following code placed into the Sheet1 code module will protect a cell
if it is in that range and you have just entered into it a new value.
If you really need to change that cells value you would have to remove
the sheet protection first.
Before the code will work as expected you will have to unlock all the
cells on that sheet. Select the entire sheet by clicking on the grey
space above row 1's row number and just left of column A's column
label, then go FormatCellsProtection de-select Locked.

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column 3 Or Target.Row 20 Then
Exit Sub
End If
Me.Unprotect
Target.Locked = True
Me.Protect
End Sub

To get the code into place..

1. Copy it
2. Right click the Sheet's sheet tab then select "View Code" from the
popup.
3. Paste the code
4. Return to the worksheet by either Alt + F11 or going File"Close and
return to Microsoft Excel"

If you use this method your Security level will have to be Medium.
ToolsMacroSecurityMedium. Also, everytime you open the workbook you
must click "Enable Macros" on the "Security Warning" dialog

Ken Johnson