Best Free anti-virus software in the internet

Hi,
I just want to know what is the best anti-virus that I can download for free.
I'm really concern with the security of my computer and I just don't want to download anything that I can see in the internet.
What I want is an advise from an expert like you guys.
Also, how would you know if your computer is already infected with a virus and what is the remedy for that.
Thank you.
