Most of the technical people on here recommend Microsoft Security Essentials which is free. I have no knowledge of it.
I use a Norton product as well as I am quite happy with it. I can usually get CD in shops about half the cost of updating on line.
If you do change to something else you will probably need to download the Norton removal tool to be sure that you have properly got rid of it before installing the new software.
Someone on here recommended MSE to me a bit ago.I installed it and it slowed my computer right down Ten minutes later I got rid!.I bought Norton 2011 from W.H.Smiths and did what it said on the box...Quite easy...
Ten minutes and you abandoned MSE....Not a fair test in my mind because there could have been a temporary slow-down on your computer for reasons outside of MSE.