The bouncing ball algorithm will do precisely that, bounce a ball within preset boundaries. The algorithm is absurdly simple. The direction variables are set to one. This is added to the coordinates of the ball until a wall is hit. Once this happens one of the coordinates becomes a -1. If it hits the wall again it becomes 1.

Here is the code:

Private Sub Command1_Click()
'Setting the coordinates
Xcoor = 16
Ycoor = 16

'Ball direction. It works like this:
'if it is 1 then the ball moves up and right
'if it hits the barrier it changes into -1 and
'reverses direction either horizontally, vertically
' or both
dX = 1
dY = 1

Do
    DoEvents
    'If the *wall* is hit reverse the number
    If Ycoor > 325 Or Ycoor < 1 Then dY = -dY
    If Xcoor > 525 Or Xcoor < 1 Then dX = -dX
    'These variables are used to erase the previous ball.
    oldxcoor = Xcoor
    oldycoor = Ycoor
    'Increment position
    Ycoor = Ycoor + dY
    Xcoor = Xcoor + dX
    'Draw ball
    Picture1.Circle (Xcoor, Ycoor), 4, , , , 0.5
    Picture1.Circle (oldxcoor, oldycoor), 4, RGB(255, 255, 255), , , 0.5
Loop

End Sub