Option Explicit
Dim oldx As Single, oldy As Single
Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
If oldx = 0 And oldy = 0 Then
    oldx = X: oldy = Y
    Line (X, Y)-(X, Y)
    Exit Sub
End If
Line -(X, Y), vbRed
End Sub