My favorite antivirus is Avira. The interface is simple, it doesn’t use too much resources, and it has a high detection rate. The security suite not only includes the antivirus, but also their firewall, which works pretty well. Normally, you can get a free 1 month license to beta test their security suite. However, they ocassionally give out 3 month licenses (and even 6 month licenses sometimes). To get a free 3 month license go Here. To download the actual security suite trial, which is around 34MB, click Here.

Edit: Looks like the old method of getting these 3 month licenses has died. No worries though, as there is a new way! Sign up for a license from this Avira promotion page (it’s in Russian). Use Google translator to guide you if you’re having a hard time.