The paid versions of products like Avast and AVG provide additional features beyond the free versions. The free versions are introductory products with limited functionality. It's not the case that they're free because they're not any good.
The biggest mistake people make is assuming that buying the right AV software will keep them safe. You have to practice safe computing. In addition to the recommendations in
this Microsoft article, I recommend using Firefox 3 rather than Internet Explorer for everyday browsing; only downloading freeware/shareware from trusted sources; and turning off HTML display/automatic message display in your email client.