I used to use AVG free a while ago on my old XP desktop until it found explorer.exe to be a 'virus' and removed it But like everyone else said, it found viruses or trojans in everything, even those that were clearly not. Also seemed to hog resources quite a lot.
Now I use Norton internet security (which I too 'purchased' ) w/ Comodo for firewall on all of my computers, and so far they work fine.
I'm sorry I'm very careful with where I put my textbooks, and I try to never let them out of my sight no matter where I am. I can't afford to be losing one of my $200 books >_<