Under Control Panel/Appearances and personalization/Display

Every so often it changes on it's own from smaller/default to medium setting and I have to change it back. This is a new pc we just put together a few weeks ago and it has happened twice already. Any ideas as to why it changes on it's own? No games have been installed on the pc yet and all we do on here is go on the web to a few web sites and play the games that come with win 7. Thanks in advance.