When you say "shrinks the screen", do you mean the window zooms out, the text gets smaller? Using CTRL + the scroll wheel (or the scroll area of a touchpad) will zoom in/out of the text on most programs. Make sure CTRL isn't being used or stuck somehow. Another culprit could be the use of gestures or tapping which I personally find annoying and can cause weird things to occur if you're not familiar with them. You can go into the Control Panel and in the Mouse options, you can turn off gestures and tapping. Also in Control Panel, check Accessibility Options and make sure you don't have anything unusual set.
You may want to consider seeking the control panel and seek the mouse option. When you are in that screen you have the option to turn off certain options which may make your screen seem to minimize. You may also disable the mouse and just a use a portable one if this option is cumbersome for you. The mouse can be very sensitive even if you are tapping on the mouse inadvertently. Try these options and you should be just fine.