Curser engages and makes squares on screen always when moving it

No matter if I am using the track pad or external mouse when I move the curser it creates a highlighted field/box on the screen. When passing over files or folders it selects them all

I upgraded to El Capitan a few months back, don’t know if that has any bearing on it.

Clicking on any close button does nothing.

I have tried resetting PRAM, using different keyboards and mouses with no change.

Yes I know it’s old, but never had a problem till now. Love to get new but $$$.

Thanks