A lot of programs will bundle******with them. Either they will have a checkbox, or a spurious "accept/deny" button. Some download sites will package software with a "wrapper" that brings things in addition to the requested program.
If you are Windows-savvy, there is a tool called Autoruns that is useful for seeing what is running and putting the kibosh on unwanted programs starting up.
It might not hurt to check your installed programs. You might have something in there called something similar to "game-saver" or "super-saver" that adds add-ons to Web browsers.