This might help get you started, paste the code in your declarations section, and play around with the form's drawStyle property to change the appearance of the box.

Code:
Private drawing As Boolean
Private startX As Integer, startY As Integer

Private Sub Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
drawing = True
startX = X
startY = Y
End Sub

Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
If drawing Then
    Form1.Cls
    Form1.Line (startX, startY)-(X, Y), , B
End If
End Sub

Private Sub Form_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
drawing = False
End Sub