Vista has a firewall that works both way. XP and earlier version of Windows didn't. That's when Zone Alarm played an important role.
Norton, depending on what you purchased, has a firewall. Norton is a resource hog and slows down your computer. It's your option on which firewall to use, Vista or Norton's.
Windows Defender comes with Vista. It seems to work well. Windows Defender also slows down your computer. I tried using Windows Defender with XP and since it was slowing down my computer, I decided to un-install it. Unknown if you can un-install this with Vista. I would consider disabling it.
I setup my sister's computer. She has Vista. I added Avast Free home edition as her virus protection. I left everything else install, including Windows Defender. She uses the Vista Firewall. I never really got into the spyware issue with her because she only reads her emails and does limited things. I did go to Windows Updates and installed all the recommended updates for her. I gave her the latest Java. I then told here to go here and download the Update Checker. Had her update the FREE items. If you pay for the items, I told her to let those programs update on their own. If you update a paid program, you may have to pay over again.
Hope this helps.
I just got a computer with Vista pre-installed; I also installed Norton Internet Security. According to Norton, I should not be using Windows Defender AND Internet Security. I'm not sure how (and if) I should undo the Defender options. ALSO, I have always used Zone Alarm and loved it...but I figured since I have a firewall installed with Norton I shouldn't download Zone Alarm. Is that correct?