Screen magnification

I have two year old mac mini and am using a Samsung monitor and a microsoft wireless keyboard 800 and wireless mouse 1000. I recently upgraded to Mountain Lion and have the weirdest thing going on.

I will be doing nothing and look at the computer and the screen will have zoomed into huge. Sometimes it corrects itself and sometimes I have to reboot.

I have nothing turned on in the accessibility settings.

I am wondering if the keyboard and mouse which have no native drivers could be playing a role?

Any help or other questions would be wonderful.

Debbie