or you can get free version of avast antivirus from
http://avast.com/and yes, it's also free like AVG or Avira AntiVir but just remember you'll have to fill out a form on their website to obtain a key so that the AVAST guys know you are using their free edition.
(But in any case, pick the one you like and ONLY HAVE ONE on your PC).
I've personally never had problem with AVG, AVAST or AVIRA. Have tried all, they all do fine. Just I liked AVAST interface over AVG after a bit of playing around with the three major free antivirus softwares and it's here to stay