I have heard that comment a few times. I have Norton installed on my laptop and it hasn't slowed it down at all. It does say on the 2006 box it requires xp and the minimum specifications have been put up to deal with the new 2006 version. Harddrive space to me is nothing as I have alot.
I have had Avg and other free versions of Anti-virus but think their rubbish. I prefer using my yearly subscription of Norton. I notice a big difference in Norton to Avg