Yes, it is
Yes, the Avast Free version is completely free for non-business use. You only need to register the use of it.
Do you ask why? Well, it has to do with knowing how many users there are so that there are a sufficient number of servers to supply all users with timely updates to the program. For virus updates, this can sometimes be several times a day. Unlike most other free AV programs that will update once a day, Avast will update as many times a day as need be for your computer(s) complete protection.
Avast Free will update the virus definitions and the program itself as the need arises providing one does not change settings that will effect this function. Avast Free installs to optimum settings and usually should not be changed.
I hope this helps you,