AVG is ok as freebies go. Avast is another to try.
I always go for freeware when I can
Though when it comes to viruses, if you can afford it, then maybe worth paying.
Nod32
http://www.nod32.com/ is one of the best paid for options. It's low resource intensive & stable. If you are prone to d/l potentially dodgy files then worth it.
Kaspersky is another good one, esp. at catching trojans too, but more resource intensive. (For a free anti-trojan, try Ewido)
snowcrash