This is almost always a virus. Many sites will block this kind of inline advertising but Giant Forums don't, which is potentially why they're only showing up here. Get your virus scanner to check, go through any browser plugins that you're not familiar with and get rid of them, and you can also sometimes tell the "Provider" of these to stop doing it, so you won't get them any more. It's also possible to get another add-on that will get rid of them, but that doesn't actually get rid of the underlying thing that's doing it.