The internet isn't free. Most web sites turn to advertising to generate the money needed to stay on the internet. Many programs have been made to include advertisements to support the people who created them. As this evolved, advertisers became more and more devious as to how they delivered their ads. Now some larger companies attempt to track your use of the internet, and create programs where the main, if not only, objective is to load your computer up with advertising. Many times these programs are secretly installed to your computer while you are, or think you are, installing a program of use.
How to fix it: