for windows its about 4 - 5 layers some you don't even realize are there and even Mac system have at least a couple layers
1st layer - router hardware/firewall
2nd layer - windows firewall
3rd layer - Anti virus program (nod32)
4th layer - anti spyware (spybot)
5th layer - UAC (user account control -vista) or with ubuntu its the authentication and the password input.
Linux and mac systems would have #'s 1,2 and 5
now my Nod32 uses up around 10mb of RAM on none scanning times and you usually never have to scan with NOD32 since it's really really good at picking up viruses
Spybot uses around 70mb of RAM since it has the "Teatimer" protection which you have to allow decisions that change your registry
these are a very small amount of resources for a pretty safe computer and another measure of protection is after the router you could always put up a hardware based router firewall or netnanny type device that can block sites and the such
http://www.newegg.com/Store/SubCateg...name=Firewalls