If this is showing up whenever you go to any website, then it is almost certainly spyware on your machine. In either case, you'll be well-served by scanning for spyware on your harddrive. While I haven't used it for years, I was very happy with LavaSoft's Ad-Aware.
This seems like the latest version of the free edition.
http://www.lavasoftusa.com/products/ad_aware_free.php