Why do the google search results lead me to advertising/advertisement sites and how do I stop it?

Whenever I search something in google and click on the link result, it gets redirected to advertisements such as enterfactory.com, asiadevelopment.com etc. Can you help mer get rid of this? It is irritating. I am using the antivirus "Microsoft Security Essentials" which also malfunctioned after manually trying to find out how to stop the advertisements. I'd really appreciate useful answers, thanks.