Because avast is effective and light? If people tell you that paid is better, free cannot protect your computer, that person is talking rubbish. Behold my "free security suite" :

-avast! Free Antivirus v6 (few minor tweaks)
-Comodo firewall v5.5 (put it under training mode first to let it learn my system)
-Malwarebytes free v1.51
-ESET Onlince Scanner
-Sandboxie free v3.54
-Neo's Safekeys v3
-Lastpass
-Use Panda USB Vaccine portable
-Fully updated flash + java
-Fully updated Firefox + Ad Blocker Plus + Web of Trust (WOT set to very effective)
-Fully updated Google Chrome + Ad Block + WOT (set to very effective)
-Run my browser and any pdf files sandboxed
-Google DNS
-Do an image backup at the beginning of the month using Macrium Reflect

Addtional services used :
-Virustotal
-Url void
-Long url

Extras :
-Keep myself inform on what is going on in the security world.

Been running the above set up for nearly 1.5 years and I never have any issues. Heck, in fact, I help to clean my friends pendrive that got infected. So far, the only security setup I have run is the above and KIS. These 2 never disappoint me. KIS is for my one of my family member which is not computer savvy at all.

I am a person who put speed and protection above anything. Those set up are the lightest, easiest and mostt effective to use without giving me problem or slowing my computer down. I have ditch Kaspersky 1.5 years ago because it became too heavy and switch to avast v5 (then). The 2012 version is still heavy. I tested both avast and Kaspersky. What Kaspersky could detect, avast could detect as well. Samples were bought back from school. So if a free product can do what a paid product does or maybe even better, why not? The time of paying for security program for myself is long gone.