static extern bool ClipCursor(ref RECT lpRect);

Private Declare Function ClipCursor Lib "user32"(ByRef lpRect As RECT) As Long

http://windowssdk.msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/resources/cursors/cursorreference/cursorfunctions/clipcursor.asp

ClipCursor(ref rect);