Ive been using AVG free and windows xp firewall for nearly a year, the pc is on the net every single day and i havent got any nasties

Honestly mate, all of this "oh my god i need a really good AV or my pc will melt down" is just all bollocks, its just rumour that spreads to make you buy things you dont really need

Dont visit shitty random/dodgy websites and you will be fine, and make sure you always keep windows up to date and your web browser, and also scan once a week with spybot/adaware programmes