The Clip property of the System.Windows.Forms.Cursor class represents the rectangle within which the mouse cursor is confined, or Nothing if the mouse can move over the entire screen. This rectangle is in screen coordinates, so you must do some conversions if you want to confine the mouse to an object on the form, or the form itself. This task is made easy by the ClientRectangle property and the RectangleToScreen method that all visible controls expose:
' Clip the mouse to the TextBox1 control's client area.
Cursor.Clip = TextBox1.RectangleToScreen(TextBox1.ClientRectangle)
' Free the mouse cursor.
Cursor.Clip = Nothing