Try Windows Live OneCare
I've been testing it on one of my PC's since January this year. Am so pleased with it that I bought and installed it on the other 2 PC's.
It includes everything you need for internet security and does not slow the PC's down.
If you should decide to try it, then uninstall Norton 360 before you download it otherwise Norton will stop it downloading correctly.
If you would like some help then use my email envelope.