When I point at something with the mouse I get

That icon means your trying to do something your not allowed to.

When you click on a link and you accidentally move the mouse at the same time you give the site the impression you are trying to drag the link which obviously you can't do, so that icon pops up.

I occasionally get it if I flick through stuff a bit fast. It's also because you are trying to do two things at once very quickly like not giving enough time for the first action to be completed.

That's about the best way to describe it. It's nothing to do with viruses or anything like that. The stalling is your computer just trying to sort out your first action.