How can *any* AV company properly defend Windows?
I used to be a Norton customer, going back many years (off and on), and at one point I was actually quite happy with Norton - but not anymore. I'm not a Norton-hater, but I just don't use their products anymore.
Some of the blame for Norton's problems can no doubt be attributed to Microsoft for creating an OS that seemingly can't be adequately protected regardless of users' safe-computing habits (not running as admin, not visiting dodgy sites, not using IE, using Firefox w/NoScript, etc.). Thus causing a growing number of users to abandon Windows altogether, for some other OS for which Norton offers no AV apps. Maybe Norton should get into the Linux AV market ;) to recapture some of that lost revenue ;)
And/or blame the alleged "organized-crime" highly-motivated black-hats who are trying really hard to make money for themselves by compromising your PC (botnets, spam zombies, whatever).
Minor blame goes to Norton itself for irritating buggy products, although their current products are much better than some of their older stuff, IMO. (At least NIS 2009 didn't use 100% CPU, however I'm not sure NIS 2009 actually *protected* against very much, either, if the Norton forums are any indication.)
Bottom line: I just don't think ANY company, Norton or anyone else, is up to the task of defending modern computers. Or, at least, modern Windows. Not so sure I trust Linux (or modern Macs) very much either, with the possible exception of certain Live CD's that have been set up for better security, but even so...