The "best" free way is to not connect to the internet at all, that way your computer will never be infected. That is unless you infect it from a CD/DVD or other storage device. You'll save money on internet service too.
Personally, I use Avast! (free version) and Malwarebytes (free as well) to scan every now and then.
Other things to do:
1. Do not open unknown e-mail, especially if there are attachments.
2. Do not visit illegal porn sites. Legit, retail porn sites are fine 'cause they don't wanna scare away customers.
3. Do not visit "warez" sites.
4. Do not install cracked games and software. Buy retail games/software.