I also give my vote to Avast. All antivirus slows your PC down a bit, any program that runs in the background does.
Topic is out of the scope for Microsoft!! Anyways in my openion every antivirus slow downs the CPU speed. It depends which antivirus you are using. My laptop feels lighter with Avira Antivir..Other eats your CPU speed
I'm using NOD32 and it didnt take too long time to load on start up. it run much faster than other compare to norton and Mcafee.
You had better check your task manage and regedit. Many tools may be good for your computer,Eg icesword and so on.
I used nod32 on my computers and I didn't had any problems with it. Updating it every day, and he is finding all viruses. I recommend it.
Norton has always been high CPU consumer right from pentium era. take any editions. It should run in background and should consume minimum resources/memory. but bad. Thread starter is right most of the antivirus eats memory a lot. I use AVG. it's OK not best still.
If you mentioned just Antivirus and Firewall software, Nod32 or Norton 2009 would be the one of the best picks. They both do not reduce your computer speed.
Every single antivirus slows down your pc, it can be super easy on system requirements, but it slows down your pc,but that mostly depends on your computer than any other big factor,but atleast for me a antivirus is a must have tool