1) Virtual extended driver 
VxD is the device driver model used in Microsoft Windows/386, the 386 enhanced mode of Windows 3.x, Windows 9x, and to some extent also by the Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02 (and higher) multitasker (TASKMGR). VxDs have access to the memory of the kernel and all running processes, as well as raw access to the hardware. == Design == Th...
Virtual Extended Driver [Microsoft]