As a computer repair tech working, I often see customers computers with lots of unwanted programs, toolbars on their computer. Because when customers install a program they are not always paying attention to the screen they click next and next again and go right by it. A lot of free software includes third-party applications to help pay for their free software, that will include browsers, toolbars and changing to their home page. By default it is already checked off by third-party software.
The only way not to install third-party applications is to go slow when clicking next, uncheck all boxes for the third-party applications. The only thing you can do if you did install third-party applications is to uninstall it with a program I use called Revo UnInstaller.
When installing software take your time and make sure your installing it correctly. If not you will have a lot of unwanted toolbars, unwanted browsers and your homepage changed that you did not want.
Here are samples of third-party applications trying to install on you computer: