WIN.COM is the executable file used to load versions of Windows that run from DOS. In Windows 3.1 and its predecessors, it is executed either manually from the DOS prompt or as a line in AUTOEXEC.BAT. In Windows 95 and onward it is automatically invoked by IO.SYS after AUTOEXEC.BAT is processed. The file is present in the SYSTEM32 directory of some NT-based versions of Windows (such as XP and Vista) for backwards compatibility purposes.
Corruption or deletion of WIN.COM caused many issues for users of Windows 95 and 98, and some early Windows viruses targeted WIN.COM. Corruption of WIN.COM would cause an error, and a Microsoft support page is still available showing information about WIN.COM errors.