One of the most common methods for malicious software to get installed onto your PC is by “piggy backing” on legitimate software. Often if you download software from anywhere other than the original software developers site you will find unwanted extras, such as “Adware” or other browser add-ins, have been installed without your knowledge.
You should always pay attention when installing software because often, a software installer includes optional installs, such as adware. Be very careful what you agree to install and always try to download from a trusted site.
Always opt for the custom installation and deselect anything that is not familiar, especially optional software that you never wanted to download and install in the first place. It goes without saying that you should not install software that you don’t trust.