I have found in the past when fixing other people's computers that AVG dosen't pick up too well on trojans. Personally, I use Avast Home Edition, and I cannot praise it enough:
http://www.avast.com/eng/download-avast-home.html
You do need to give them your e-mail address to get a licence key, but it's free. After that, your antivirus can be forgotten about - it will update itself automaticly. If asked to, do a boot time scan - this will search for viruses before Windows has even started. It even stops viruses downloading before they are even completley downloaded. But that's enough of me saying how good it is, why not try it for yourself?