I had an afterthought on this and wondered if you might appreciate a little
more information if you need to delete the comment or edit the info in an
existing comment. (You can't add a comment if one already exists.)

Private Sub Worksheet_Change(ByVal Target As Range)

With Target

If .Value < Cells(Target.Row, "B").Value Then
If .Comment Is Nothing Then
.AddComment
End If

.Comment.Text Text:=Application.UserName & ":" & Chr(10)
.Comment.Visible = True
Else
If Not .Comment Is Nothing Then
.Comment.Delete
End If
End If

End With

End Sub

--
Regards,

OssieMac