Can other digital equipment too close to iMac cause damage?

I was curious if anyone knew if other digital equipment (like a cable box) can cause damage to the iMac graphics or any type of damage for that matter? I ask because I have a cable box right above my Mac. I had problems with my last iMac where the graphics went out, then eventually the whole computer just went out on me. I have a new iMac now and this morning I went to log in and the graphics began to act funny, everywhere I dragged my cursor on the screen left a trail if grey behinnd it,almost looked as if I were playin the "draw something" game. I re-strated my comp, and so far it's working fine now. Can anyone help?

Thanks!