Google Redirect

Hi, When I click on a search result from Google, it redirects me to another website, usually advertising for something. Also, sometimes on these redirected pages a red pop-up box that says 'page attack' or 'attack page' (can't remember) comes up and warns me not to go to that site. Recently I ran a scan and a couple of things were detected and moved to the virus valt, however there was something it couldn't heal because it was unreachable or something. I have AVG Internet security on my computer - the paid for version. I am running Windows XP, Mozilla for internet browsing, and have a Toshiba laptop. Thankyou