AVG is a very poor anti-virus i'd recommend doing what a lot of people have already done and switch to Avast which is also free, read below for more info:
Avast home edition is free and effective, it protects your PC from infected webpages, scans your downloads and emails for viruses (AVG free does neither) and has built in anti-rootkit and anti-spyware, plus it gives you the option to run a boot scan, there are very few anti-virus's that can do this paid or otherwise and this is free.
Avast works for 60 days from installation after which you will need to register to get your free product key which is valid for a year after which you get another free key, so it always free.
I use Avast along side Comodo firewall both are free, light on memory and have been independently tested and proven to be more effective then Norton or Mcafee.
Avast: http://www.avast.com/eng/avast_4_home.html
Comodo: http://www.personalfirewall.comodo.com/
Why Avast is better then AVG:
On AVGs official website it states that their free anti-virus software does not protect you from “poison” webpages or scan downloads and emails for viruses (Avast free edition does all 3), I've also tested both AVG and Avast using the EICAR test viruses, AVG allowed all the viruses onto the computer without a single warning Avast either blocked them instantly or shorty after preventing all from running, Avast is much lighter and offers far superior protection.
What AVG does not do: http://free.avg.com/download-avg-anti-virus-free-edition