You have probably unintentionally installed a program on your computer as a bundle of free software downloaded from websites.
When you install free software that seems useful for you, the truth is that you are probably installing some browser hijackers, toolbars or other type of malware.
Mostly, you don’t know that there is a malicious program installing on your PC and modifying some settings of your PC system and browsers as this installation runs when you keep the option—Recommended installation the free software gives you when install this free software.
Therefore, please always remember to select Customize installation and refuse any unknown programs’ installation.
To uninstall the programs that gives you the ads, check this guide.