That's like saying "I'm an STD doctor who specialises in treating and curing STDs, so I don't need to wear a condom".
Its sort of like that. Like, very like that. Except, computer viruses aren't serious and can be fixed and troubleshooted and scanned for by hand... ...so basically, no, it isn't like that at all. Those would be the same, if STDs were acceptable and easy to check for on anyone regularly (it isn't, go ahead, hook up with someone and tell them you are taking a sample of blood first off), and if STDs were treatable with a series of silly steps to clear them right up.
You don't need a free AV anymore. Microsoft Security Essentials does the job, isn't intrusive, and doesn't spam you with approximately 20 popups every time you try and install or run a new piece of software (AVG being notorious for this, and COMODO while being an excellent program still doing this too much). Speaking as someone who ran COMODO for half a decade, with no regrets.
That isn't universally the case. You can seriously brick your system. I just don't see the sense in running the risk when there are free solutions available to significantly mitigate the risk.
Microsoft Security Essentials is a scam, a continuation on their Defender bullcrap that doesn't do or prevent anything. I swear by Avira. Also, Comodo Antivirus has never been on par, I only use the Firewall part for its HIPS protection, which is pretty awesome. Been using Avira ever since I had my own PC, settled on Comodo Firewall like 5 years back. I also regularly check if it's still up to standards these days. Combined they still have everything you could possibly need these days. Zone Alarm would be too simple, Online Armor isn't really free as it locks you out of options you would really need as a paranoid user like me. Little fun fact: The Windows 7 I'm running right now is as old as my PC - about 3 years. Maybe I'm just a cautious user, but the things that slip through, either Avira or Comodo catch for me.
AVG's constant popups upset me so I switched to Avast! after having used them for quite some time. Does the job fine by me.
Microsoft security essentials, you cant still use the old trick of using an older version and patching the virus information? You used to, so my friend kept a copy of old files meanwhile. Still, I have to literally choose, between playing RenX, and running antivirus. But, I do occasionally use Malwarebytes and CCleaner.
Malwarebytes lost his trustworthiness now that they exclude PUPs as malware, given "you can opt out of them, if they get installed, you allowed them too".
Oh wow, a virus hidden in a screensaver. I haven't seen this something like this since umm windows 95