How do I get rid of this pop up?

I think my computer may have a virus. Every time I open up a new window, or go to a website (no matter what it is, even Yahoo or Google), I hear a voice saying 'congratulations'. Then this window comes up saying I am the lucky visitor who has won something, usually having to do with a survey. This is really annoying, how do I make it stop? And could it be harmful to my computer? thanks :)