About which antivirus is better

Avast! Free Antivirus Version.

This antivirus has the largest number of users in the world.
It was awarded as the best free antivirus of 2012.
Its virus database is one of the most complete.
It is compatible with Windows 8

AVG AntiVirus Free Edition.

The AVG Free Antivirus version has a style very adapted to the new Windows.
I can tell you that it has been in the market for many years, that it is used by millions of users in the world, that it is intuitive and easy to use.
It has a great detection rate.
It consumes few resources on your computer
I don't like that you change your home page to Secure Search, but this has a clear solution, to put your home page back as it was.
It has a bar that is also installed in the AVG Toolbar browser.

Avira Free Antivirus

Avira, is known as the umbrella.
Sometimes it gives some false positives, that is, it thinks there are viruses where there are not.
The downside is that it has advertising. Free stuff ...

Microsoft Security Essentials

Microsoft Security Essentials free is from Microsoft, it was created by them to defend against constant attacks on Windows.
Now it is also called Windows Defender.
They give it to you installed with Windows 7 and 8.
It is very complete, fast, offers acceptable detection rates.
If you have Windows XP, you can download it, it is also available until 2015