Screen looks like a photo negative

My kids turned on their eMac today. When it completes the start up process the screen is normally colored. At the last moment of the start up it changes to almost a reverse of all the colors. It looks like a neon colored photo negative. The screen is black in color with the accented colors a muted neon color. Is this a virus? It is almost like a start up item but I checked and no program exists in the start up menu of my user profile. This happened to my iMac a few years ago. I solved the problem by reinstalling the OS. I would like to avoid that if I could.
Thanks
Rob