Antimalware

Hello, I was watching some YouTube today and I saw some really suspicious ads on my PC like insureandgo and you have 1 sms remaining! (These were those ads on YouTube on the side of the page) I ran Avira free AV and Malwarebytes free and nothing was detected. Why? Is this happening! Thanks Ps...

On my newly built PC I want to go ahead and get some antimalware/antivirus software just to be safe. I highly doubt I'll ever run into any issues as the truly best protection is common sense. I never visit any sites or download anything I don't trust. It has worked with me for years. But I still...