When We talk about security what sort of security are we talking about:
Software, hardware, network.. all of the above?

with a PC, what are we after and where from are we protecting

1/ The complete denial of access to the PC's data, and its operational processes from:
a/ out side the room,
i/ outside the Systems Network
ii/ with network access
b/ inside the room, outside the systems Network, not in physical contact with the machine
c/ inside the room, on the systems Network, not in physical contact with the machine
i/ Guest/user access to network
ii/ Admin access to the network
d/ in physical contact with the machine, and with:
i/ only guest access to the system
ii/ with Admin access
e/ internal access to machine.. boot access

What level do we protect to..

just the data
Usage information.... user profile
the integrity of the software ie prevent user reverse engineering or cracking a program
Software profileing
prevention of running non certified or pirated software
Product branding (this is a MS computer piss off *nix ..etc)
Hardware locking.. modding/cracking

As already mentioned there are hardware criteria for security, as well as the various software many of us are familure.

Network security is another level

no matter how well secure your home, it isnt if you leave the key under the mat