I wouldn't use Avast since this company's policy with clients' data is not clear. If I am not mistaken there was a scandal with Avast not long ago about that. They were caught red-handed selling their clients' data.
Instead I would choose preinstalled solution by Microsoft -
Security Essentials. I saw some testing on the web and this free solution was not inferior to Avast, Nod 32 and some other anti-viruses. Also I recommend to install No-Script addon to your browser (if you use FF).
UPD: By the way if you need some more information about security I strongly recommend to read this company's blog
https://www.digitalforensics.com/ The other day I stumbled upon some really interesting articles about online blackmail (it is on the rise) and about the signs to see if your computer has been hacked or not.