McAfee Antivirus was installed on my recently purchased Dell laptop. Then I learned that the university where I'm located recommends Norton antivirus (though they later said both are about equally effective). I thought to be doubly safe, I'd download and install that, too, from a software suite I'd purchased for $15 from the university.
That was a big mistake. Basically, I couldn't use my computer -- everything was frozen. It took several days of phone calls for the IT people to figure out that the problem was caused by my doing this. They said the two softwares read each other as viruses.
I was annoyed with all kinds of McAfee popups on my computer so I ended up uninstalling that. I then uninstalled Norton, then reinstalled it using a free download from Norton's website. It is now actually called Symantec AntiVirus Corp., but the Norton website (sorry I don't have the URL offhand) brings you to it. There are "live updates" when you click on an icon once you have it installed.
You shouldn't have to pay for antivirus software.