avast! Home Edition is free of charge for non-commercial home use. You can find more information here. If you are not a home user, or you use your computer for profit, avast! Home Edition is not licensed for such use, and you are asked to use the commercial version – avast! Professional Edition.
To use the Home Edition, you should register it, however, after initial installation you have 60 days to trial the product (as with all avast! antivirus products) before you must complete registration. The registration process is very easy, and will take you only a couple of minutes.