If you use Avast Home, you can register it for free. You'll get the serial by email within a couple of minuts (if not seconds). The serial will be valid for about 14 months. If it expires by then, you can register it again, and get a new serial. It really is completely free. I use it for a couple of years now.
If you need a serial for the Pro version, well, then you'll just have to buy it.