I would first run a full virus scan to rule out your own system as the culprit. Go to
www.trendmicro.com and use their free online virus scanner. It is very effective at removing viruses. Overall, I trust trend micro and AVG more than Norton and some of the other paid AV software on the market. Others may disagree as everyone has a favorite.
This is just my own personal opinion and experience based on running my own computer repair business for the last 4 years.