Just stick with the free version of AVG. As dyserq said, u go and get a key from the internet, I tell u that without good protection ur pc will be full of viruses, trojan, spyware, adware and that kind of stuff.
I know a website where u will find the product key for almost any program u need, but u visit that website the same moment u close ur browser there is a javascript trojan in ur pc.
So just stick with the free version.