I've been running antivirus forever now and i just don't see any slow down but Nortons has stopped viruses for me a couple of times. Maybe if i was running an older machine, pentium 3 500 mhz or something it might be an issue. Running a dual machine, as you know yourself, i've never experienced any slow downs. I just like having the protection. Nortons aslo seem to update their definitions pretty quick. Normally when a new virus is released they have a download ready within 24 hours. If it was such a big issue with slowdowns i wouldn't use it.