Which is best Free Antivirus ?

Avast is a good free antivirus

http://www.avast.com/eng/download-avast-...

also recommend downloading a (free) firewall

I use Comodo Firewall

http://www.personalfirewall.comodo.com/

you also may want to download these free antispyware programmes

http://www.microsoft.com/athome/security...

http://www.emsisoft.com/en/software/down...

download A Squared Free 2.1

http://www.safer-networking.org/

http://www.lavasoftusa.com/products/ad-a...

Please remember use only

1 antivirus

1 firewall

you can use as many antispyware programmes as you like(2/3 seems to be enough)

Good Luck