You have to register it and enter a key, otherwise you're in "trial mode". It's free to register, but it then supposedly expires in one year (and you register again for free). I say supposedly, because I've seen the keys expire long before a year is up in some cases.

Now... I find the real time scanner in Avast to be not so great, so beware. It doesn't do so well at preventing trojans.

P.S. Register it here:

http://www.avast.com/i_kat_207.php?lang=ENG

( link from http://www.avast.com/eng/download-avast-home.html )