I used AVG Free for many years alongside some free spyware software BUT ran into problems that it would not update. Eventually got a virus I could not remove.
Purchased Kaspersky Internet Security when I got a new laptop and bought another licence for my infected PC - got a very good deal, 3 pc licence for 2 years for under £15. The best bit of antivirus / spyware / adware software available IMHO. Updates several times a day and free (very fast) online support.