I am getting a lot of the cads popping up,,dont want them and they r nasty,, how can i rid of them?

these ads pop up at any time and i have not downloaded anything to get them,,they arre nasty and i dont like or want them,, its either called dads or cads or something,,i dont pay attention i just delete them,, i want ti rid of them once and for all...please send reply to email thanks