If you have ever opened a bank account, purchased something with a credit card, or even have a birth certificate, then extremely private personal information about you is stored on a computer - somewhere. No doubt you'd like to know that computer and your personal information is secure. Secure from what? One of the biggest threats these days is Identity Theft. If a person is able to get enough of your personal information then they can assume your identity, destroy your credit, and ruin your life.
There is more at stake than just your identity, however. If you have money in a bank, fund, insurance policy, or anything similar, then what you have is in reality a number on a screen. There may be physical documents assuring the validity of what is on that screen, but there is no box in the back room with your name on it. Debt is similar - just a figure in a computer. In the manner of Fight Club, destroying that data, in essence, eliminates it's existence. While it might be nice to "lose" your debts, if the data of your bank account (or whatever it may be) were to be compromised, you could lose everything.
Truly then, data security is the most important thing.
Over the years security has become valued more and more. Thus, new ways of ensuring it have been developed. In a Windows Network Environment, security is ensured in a variety of ways.
Physical Security
It is impossible to place "too high" a value on physical security. It has been said time and time again, that if someone has physical access to your network or servers, then they can find a way into anything. Servers or devices housing sensitive data should be placed in a locked room without windows, and with no false ceiling. There should be no way for an unauthorized person to get inside, and the list of authorized personnel should be short. Access to computers and servers on the network should be tightly controlled, and users should be required to use a "two part" authentication: Something they know (password), and something they have (token, fingerprint, etc.). Data that is stored should also be secured. Many organizations store data backups on magnetic tape. A good way of securing that data is to keep it inside a room equipped with a magnetic eraser at the door. These special devices will completely erase any data stored on a magnetic device if it is removed from the room while active.
Firewalls
Firewalls can also be considered "physical" security, as they physically set a barrier between the Internet (that's where all the bad hackers are) and your internal network where sensitive data is stored. Firewalls need to be locked down to allow only the bare minimum of traffic to pass through. Stateful firewalls can also inspect the contents of packets going in and out, ensuring that no sensitive information is leaving the premises, and that no harmful programs are entering.
Logical Security
Security is maintained within the network by using things like security groups, group policy objects, access control lists, and of course passwords and authentication. Access to various file systems, applications, and databases must be restricted to a "need to know" list. Administrators need to be diligent about disabling user accounts that are not in use, explicitly setting permissions, and enforcing policies. A careless or lazy mistake can completely trump all of the other efforts of security.