You might use a worksheet event. This will undo any entry in any cell if
the cell 16 rows down is not greater than 0
Add the select to move active cell to the next cell down.

===========================================
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False

If Target.Offset(16, 0) <= 0 Then
Application.Undo
' Target.Offset(1, 0).Select
End If

Application.EnableEvents = True

End Sub
===========================================

For a specific cell:
===========================================
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False

If target.address = "$G$5" and Range("G21") <= 0 Then
Application.Undo
' Range("G6").Select
End If

Application.EnableEvents = True

End Sub
===========================================
--
rand451
"Billparsons40" wrote in message
...
I want to allow users to enter a value in a cell (say G5) only if another
cell (G21) has a value greater than zero.

If G21 is zero or blank, then I do not want the user to be able to select
G5.

This is for Microsoft Xcel 2000.