A site you visited previously, or in another browser window, may have launched a "pop-under" ad which appeared behind your open windows and only later became visible. Some free programs (not My Little Mole!) secretly install hidden ad serving programs when you download their product. These ad serving programs often cause pop-up ads so that it looks like they're coming from websites that don't actually serve ads. You may want to periodically go through the programs installed on your computer and remove the programs you do not recognize (Start > Settings > Control Panel > Add/Remove Programs). You should also use anti-virus software and anti-spyware software which will remove these programs. Be careful! Some sites claiming to offer anti-spyware or anti-adware programs are actually fraudulent and will install ad-ware or spy-ware on your computer.