Normally, you use a Windows or MacOS workstation locally. In other words, you are sitting in front of the computer, typing at the keyboard that is connected to the computer, and viewing the monitor that is connected to the computer. Windows 95, 98 and ME, and the MacOS 7-9, all let the local user do pretty much anything with the system and the files stored on it. If you share the computer with someone, there are primitive methods implemented to prevent you from deleteing another person's files, but they are easily circumvented.
Systems with emphasis on networking though, like Windows NT, its successors Windows 2000 and Windows XP, and MacOS X, are designed to be multiuser and to allow remote logins. In other words, as a user connected to the internet, a cracker can send a connection request to your computer and get the login dialog box, just as if the cracker were sitting at the keyboard. From this point on, all the cracker needs to do is guess the right combination of username and password and the system allows the login.
Windows NT has had countless remote login exploits, and so have 2000 and XP. Unix-based MacOS X implements the same sets of file permissions as traditional unix, and is much more secure "out of the box".
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