It seems the latest trend for annoying users of the web browser is to create pop-under ads which are typically launched with a simple mouse roll over. You no longer have to click on the ad to launch them and what is even worse is the ad typically pops up under the current window you have open. You may not even know about it unless the ad contains audio, which they usually do. As a recent client contacted me in a panic, thinking someone had connected to her computer since she heard someone speaking, and immediately shut things down figuring by pulling the plug would prevent this.
So, with a little knowledge of just what is going on and recognizing that it is quite a common thing to have multiple windows and/or tabs opening on any given web browser (Chrome being my choice as the one to use and highly recommend it as well), just dont forget to minimize the current window to see just what may be happening in the background. You can also RIGHT CLICK on the current tab you have open and choose to close all other tabs. If a new browser window opened and you are using Windows 7 then you should also be able to move your mouse over the program icon on the task tray to see the other windows that are running and choose to close them by clicking on the red X.