Center Box Pop Back Up Screen Interupting

My antivirus that came installed on my vista computer expired I guess because I kept getting allthese notices aobut it, so I clicked on i and it took me to a website to buy anitvirus 2009 for 50 bucks. I did that and got a registration key and entered it and everything seemed ok. But then aobut every 15 minutes or so, the security center box will pop back up on my screen interupting whatever I am doing. If I am typing, it minimizes my paper and brings up the box. If I am playing a game, it minimizes the game to show me the box, which often results in me becoming dead. What is goig on and how can I stop this.