yep AVG free is good if your after something free(ive used it in the past)
Avast ive used but only for literally 5mins because i hated the user interface and nothing made sense to me!

If you dont mind spending abit of cash on something better then i wouldnt look any futher than Nod32 http://www.eset.com/ ill let the website do all the talking but basically this is what ive been running for the last 5months and it has been faultless and it doesnt bog down the pc like norton and some other Antivirus's do

Nod32 is just an Antivirus/ scanner(read on the website for its exact purpose) so you need a firewall aswell, id recommend zonealarm, im using the Pro version which has some cool extra features and does its job very well
(Most all-in one suites with AV+firewall etc etc arent very good)

Any questions feel free to ask