Automatic Start Screen comes on

Hi everyone,
I hoping someone might be able to help me. I have been using Windows 8. Very often when I am on the internet or on the desktop screen and I move my finger on the mouse pad, the Start screen automatically comes on. This happens when I move my finger on the mouse pad and the screen automatically goes to the Start screen. It is very annoying. Can you please suggest something that would prevent the computer screen moving to the Start screen even when I do not want that screen.
Thanks in advance for your help.

Where is the mouse pointer when this happens? If it's off to the top right side then it could be that you are opening the Charms bar and selecting Start accidentally.