When you are using your favourite browser to access a website, an advertisement appears in a new browser window even though you haven't clicked anything. What is the problem here?

Pop-ups are annoying, but they aren't always a sign that your computer is infected. Most likely, a pop-up advertisement will appear because of programming on the website. Adware pop-ups typically swarm your desktop with a number of windows at once. Viruses typically cause more harm than a simple pop-up advertisement, whereas spyware typically doesn't announce its presence.