I also heard that Norton slow down system a lot I am using AVG with zonealarm since long time without having any problem. DON.
Well, maybe that depend on the pc itself. I used NOD32 and have no problem (in term of slow down and such) and my sis used AVG for her laptop, it also has no problem.
Norton/Mcaffe (or whatever it is) are both system hogs. Try NOD32, it takes very little system and is on the whole more effective that these monopolies. Better yet use Linux
But personally I love the McAffe you can get it for free if you sign up and create free account with AOL. Free for one year. Using it for 1 year plus and until now cause me no headache
Norton Antivirus is probably the antivirus software that would use up the most resources on your computer. It doesn't even catch all of the viruses that are on your computer, overall it is junk. If you want to go for a GOOD anti-virus that is FREE, then go with AVG. If you want to go with a BETTER anti-virus that cost money, then go with NOD32.