AVG


Pros:
+ reliable software developer
+ basic version of antivirus is freeware
+ rather high detection capabilities
+ free updates

Cons:
No noticeable downsides. But beware, that NONE OF antivirus programs could guarantee 100% protection of your computer. And remember one more thing: ANY antivirus SLOW DOWN your puter. More or less.

PS I prefer not to use any antivirus applications on my laptop. But i am using trusted sites on the internet and from time to time check my PC with on demand scanner. Hate those residential thingies. 

Download:
PC version
http://free.avg.com/us-en/download-free-all-product
Android version
https://play.google.com/store/apps/details?id=com.antivirus

PS Installing antivirus on android is optional, because quantity of viruses for this platform relatively small, so i don't bother with installing. But if you are a easily carried perfectionist....