I dont run antivirus, just have a firewall and im SMART about what i do, 6th year running on the same non-reformated drive, and im fine still runs fast.Well actully i take that back I reformated once but that was because everything was all messy and in like 9 partitions. and the main things were my games, which i can download though steam
nothing else was infortant, or else it was saved on a CDROM. that was like 2 years ago. I actully had norten once, it was a pain in the behide, with all of its messages and crap and it did make the PC run noticibly slower.