https://www.avast.com/en-ca/index This is the one I use; works rather well and has detected/removed quite a few viruses for me over the years with it's free service. Download Link for the free one: http://download.cnet.com/Avast-Free.....t&ls=media

Yep avast is one of the most popular free and avira is good to i think

Io uso Avira e mi trovo molto bene.

I agree, Avast is pretty good, you could also try Malwarebytes, i tend to use it as well and it's pretty good too and free as well

Malwarebytes anti malware è il migliore accoppiato con windows defender :p

I use Comodo myself - their free anti-virus / firewall suite is pretty decent.

Also recommend:
Spybot Search and Destroy - to find and remove malware and immunise your system against getting known ones installed.
CCleaner (good for cleaning out junk tmp files and such)