How can I stop being redirected to ad sites ?

[Rach]

It's some kind of virus or something. Sometimes when I click on a Google search result I get redirected to a random advertising site. I then click'back'and once I click the result again it usually takes me to the actual site. Its extremely annoying and I would like a way to fix it. I would much prefer it if I don't have to download anything to fix it.