I have been using Avast! Home Edition Antivirus for nearly 8 months now and I am really impressed by the product and the best part is that it is totally free to use you just have to register with Avast! to get your 14 months of subscription for free and after your 14 months subscription is over you can just renew your key for free. Here are some key features of Avast!:
Free to use(Home Edition only)
Small Update Size
Average scanning speed
Provides various protection shields(Resident Scanner, Web Shield, P2P Shield)
Scans incoming/outgoing e-mails(only if configured with a e-mail client for e.g Outlook Express)
Customizable Skins(This thing is only available with Avast!)
The above mentioned features are also available with any paid AV Software but why to pay if you get all of them at no cost. The only thing I feel it lacks is a FireWall but there are thousands of freeware FW’s and they can downloaded from sites like softpedia.com. If you can’t afford buying a AV software I strongly recommend you this its the best.
You can also get this editon(HOME users only) here:
https://avast.com/eng/download-avast-home.html
This is the best AV software I have ever used.