A SYS - Windows System File is a file type that is used by both Windows and DOS. This particular file type contains settings, variables and functions that enable the operating system to function properly. SYS - Windows System Files are commonly used to store device drivers and other core functions. Some common files of this nature include Io.sys, which stores a computer's default DOS device drivers and DOS start up program, msdos.sys, which houses the entire core operating system code for DOS and config.sys, which DOS uses to store all its primary configuration information. These files, along with other well known and little known SYS - Windows System Files, are usually found in the Windows directory or Winsxs directory of various versions of Windows.