I use AVG Internet Security 8.5 (not the free edition as it's not that good tbh) and it really is fantastic and won't slow your pc down at all, my pc came with mcaffee which was the worst thing i have ever used so that got uninstalled, I then tried Norton 360 and that slowed my pc down very badly so then tried AVG 8.5 and liked it so much I bought a subscription for 3 pcs, (my pc, g/f's laptop and parents pc) for about £25 for a year.
Also worth noting, My mum and dads pc had 11 viruses (yup, they click anything lol), norton and mcafee only detected 3 and 5 respectivily, norton could only remove 2 of the 3 it found, AVG found all 11 viruses and a load of spyware and got rid of it all, their pc has been fine ever since.
AVG 8.5 is a LOT better than the free AVG, give it a try as there is a 30 day trial :)
