Windows XP also defines a number of computer-related variables by default. For example, the %windir% or %systemroot% variables can be used to access the directory in which Windows XP is installed. Although this is usually C:\WINDOWS by default, it doesn’t have to be. On systems with multiple operating systems installed, this variable makes finding the current Windows installation folder a breeze, and eliminates guesswork.
Other computer-related variables that can come in very handy include %computername%, %systemdrive%, and %os%. The %computername% variable returns the name of the computer, %systemdrive% can be used to represent the active system drive (typically drive C), and the %os% variable will display the name of the operating system in use. When using %os%, this variable will always return the value Windows_NT on any system running NT, 2000, XP, or 2003, but this can be changed if you prefer. The main purpose of this variable is to distinguish NT-based systems from DOS-based systems like 95/98/ME.