Your PC is quite safe, the banners at the top are either kept on the pprune server (reliable for the forum) or they are on a third party server.
Aerodynamics for example design their banner and save it to their server along with their website files, they then advertise on pprune by just giving the link that we then put into the web page code. But as you've seen, if they need to reset their server or it goes offline then you have no advert
