Pop-up windows are used by advertisers to annoy internet surfers. Each time a popup window opens, it slows down your surfing because not only does the advert have to download, but your computer has to load a new instance of your web browser into RAM. Now that even Google has allowed adverts of this kind, I recommend this little freebee from download.com:

Popup Stopper by Panicware

It works great. In fact it never fails. Whenever it blocks an pop-up ad your cursor briefly flashes into a red-circle-with-a-line-through-it symbol, then back to normal again. I no longer have to stop surfing to close all those pop-up windows. Enjoy!