Let me add my two cents...I use "Panda 2007 Internet Security" on my system.
It runs pretty smooth for me; and it's not a resource hog like Norton...
(Stay away from these type of programs)

For system defrag I use "Diskeeper Workstation", don’t get this confused with Windows Defragmenter, they look the same but they don’t work the same. The copycat Windows version is very slow and sloppy.