Thomas,
Paste the following code in the code pane for the worksheet (press ALT+F11
in Excel and click on Sheet1 in the VB Project Explorer):

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = Range("B1").Address Then
If Target.Value = "YES" Then
If Not Range("A1") <= 0 Then Range("A1") = Range("A1").Value - 1
End If
End If
End Sub

This should do it.
Good luck.
E. Froma