First download and install Sbybot from any of the buttons that say download http://www.safer-networking.org/en/mirrors/index.html

 

Run the program and select search for problems, give it a few minutes to run through your computer and when it offers to fix the selected problems click to clean up the mess. I run my program about once a week. If the problem is REALLY bad you may need more advanced help, mine is still having problems and I am not sure what to do next.

 

Your second step is to prevent them from coming into your machine by downloading and installing the Google toolbar. This only works with Internet Explorer right now http://toolbar.google.com/ This will block the ads from coming in, but you still should run Spybot once a week or so.