As the popularity of the digital Web-cam¡Athese devices are more and more cheaper and powerful. We can apply computer vision techniques with camera and projector to build a more convenient presentation system. In presentation, sometimes due to the position of projector, the images appear the perspective distortion (keystone distortion). The user should manually adjust the position of projector or use the keystone corrections of the projector. But when the distortion is not trapezium, the built-in keystone corrections are not suitable in this situation. We present a computer-vision based method that uses a Web-cam to calibrate the keystone distortion. The Web-cam takes the images that the projector projected on the wall. If the Web-cam observes keystone distortions of the projected images, we use a geometric transform that pre-warps the images in the projector frame, such that these images appears rectangle with known aspect ratio after being projected on the wall. Besides, we implement the virtual buttons that allow users to interact with the computer. The virtual buttons means that when the camera detect the laser point is on the virtual buttons, computer triggers the event as the virtual button being pushed. This paper uses point-matching pairs to obtain the homography between camera image frame and source image frame. The homography, that is the fundamental of calibrating perspective distortions also help us to search the position of the laser point.