Don t listen to people who say "Norton slow down the machine" and "Norton sucks",they don t know what they say.
Norton has improved so much in these years and becomed one of the best antivirus .
Norton Internet Security is at the top of the Charts made by the best Labs of Malware Research.
Anyway,i recommend you Norton 360,the best over all,i use it since 2 years in all my notebooks and they run fast and safe.
Install Norton Pc Check Up too,it is free,you'll see,it is good program for your pc,it makes a complete diagnosis of your pc.