Since back in the days of DOS, Microsoft operating systems have used environment variables as a way to control the user and system environment. While originally defined in files like Autoexec.bat, the capabilities and functions of environment variables are a core element to any XP system, and can be used to manipulate or control the ways in which applications and users interact with the operating system. With an understanding of existing and custom variables, you can easily obtain a greater degree of control over how your XP system functions.
At the most basic level, a variable is a type of placeholder that can be used instead of a piece of constant information when interacting with XP. Environment variables are typically used from the command line, within scripts, and by system functions in order to return a requested piece of information. For example, if you wanted to quickly access the Documents and Settings folder associated with the user currently logged on, you could open the Run command, type %userprofile%, and click OK. When environment variables are accessed on a Windows XP system, they are enclosed within percentage sign characters (%).