To begin with it is important to state that there is no best antivirus software out there. Why is this? Well, any product that you choose will act differently against various viruses since they all have technologically different AV (Antivirus) engines and components. While one antivirus software might have a higher detection rate, another might have better malicious URL blocking or virtualization techniques, while another might have less of an impact on system performance and so on and so forth.
1. Qihoo 360 Total Security
Advantages:
• Light on resources, extremely fast scanning times and pre-configuration protection modes.
• Easy to install and comes with no adware.
• Very fast updates/fixes and excellent customer service with immediate responses.
• Great detection rates, with very high zero-day protection.
• Includes Glasswire Firewall, malicious URL protection, network threat blocking, and windows patch-up components.
Disadvantages:
• Might encounter the odd false positive.
• Speedup and clean-up tools might not be for everyone.
This antivirus software is compatible with Windows XP-10 and Mac OSX and can be downloaded here. It will only take up 41.6 MB of your memory.
2. Avast Free Antivirus
Advantages:

• Very light on the system with a modern and clean UI (User Interface).
• The only antivirus software that comes complete with a fully customizable installer.
• Top notch detection capability.
• State of the art malicious URL blocking, network protection, integrated password manager, and outdated software checking. Furthermore, it comes with a rescue disk.

Disadvantages:

• Bloated default setup and has some ads and pop ups.
• You have to create an account after a month for further protection.

This antivirus software is also compatible with Windows XP-10 and Mac OSX and can be downloaded here. This software will take up 193 MB of your memory.

3. Comodo free Antivirus
Advantages:

Has many features and has lots of customization options.
Multi-layered protection scheme with Sandbox, HIPS, Antivirus and Firewall.
A clean UI (User Interface).
Tweaked settings means that this software has one of the best 0 day protection packages.

Disadvantages:

Isn’t very beginner friendly.
Buggy software.
Updates are slow to be released.

This antivirus software is also compatible with Windows XP-10, Linux and Mac OSX and can be downloaded here. This software will take up 208 MB of your memory.
4. AVIRA Free Antivirus
Advantages:

Is light on the system and runs very smoothly without slowing the system down.
In-depth scans which produce very few false positives.
Browser safety Add-ons available for all major browsers.
Clean ad-free GUI (Graphic User Interface), Ad-free installer, and no pop-ups or ads.

Disadvantages:

Zero-day protection is very weak.
Very ineffective Browser launcher which takes up a lot of memory.
Painful removal for detected malicious files.
No firewall/sandboxing/ web shield technologies.

This antivirus software is compatible with Windows XP-10, and Mac OSX. This software will take up 200 MB of your memory. It can be downloaded from here
5. Panda Free Antivirus
Advantages:

Low memory and CPU usage thanks to its cloud protection.
Tiled customizable UI (User Interface) and nicely rendered Settings interface.
Good detection rates and behavioral analysis program.
Decent web protection and hardware resource handling.

Disadvantages:

Requires an internet connection which leads to weaker offline protection.
Slow scanning speeds and occasionally has issues with virus removal.
Watch out for the Panda toolbar during installation.
Sometimes gives false positives.

This antivirus software is only compatible with Windows XP-10. It can be downloaded here. This software will take up just 61.4 MB of your memory.

more............
http://trillionclues..com.ng/2016/12/5-of-best-free-antivirus-software-you.html

For a Windows PC, Security Essentials is good enough. Just know that things like firewall & internet security are not included in any free antivirus software