Yep! that's right. All major advertisement that you see on websites asking you to run a free virus scan, usually when you download something, are FAKE ADS. You trust them, install them and then these small programs show you that your computer is infected with deadly virus or have thousands of problems on your system even if your have purchased your computer/laptop a day ago. See how powerful they are :)
And now to remove these issues they will ask you to purchase their software.
Now the truth is, these kind of softwares are ABSOLUTELY FAKE softwares which means they do not remove any virus or malware. Once you will purchase these softwares, on regular intervals they keep poping-up that they have captured some viruses and have removed them but technically they haven't done anything. They simply show a message, fool you around and are making money.
Below I am displaying you one such ad:
Please use genuine anti-virus program like Norton, Bitdefender, Mcafee, QuickHeal, Kaspersky etc. to keep your computer free from Viruses/malwares. These anti-virus company have never displayed ads like these Fake ads.
Stay alert and be safe.