About Excel Events in excel 2003

You can try to use selectionChange event.

You can evaluate which cell has current focus and keep track of last cell range that had focus prior to the current cell range.

Private Sub Worksheet_SelectionChange(ByVal currentRange As Range)
Static lastRange As Range
    If Not lastRange Is Nothing Then
        If lastRange.Address = "$A$1" Then
            'Run Code
        End If
    End If
    Set lastRange = currentRange
End Sub

Regards