VxD 
virtual device driver 
A VxD, or virtual device driver, is a specific type of Windows device driver that has direct access to the operating system's kernel. A VxD has access to low-level data, such as information passing between the operating system and applications. The term VxD refers to a generic device driver; specific device drivers are referred to by replacing the x with the appropriate signifier. For example, a VDD is a virtual device driver for a display adapter; a VPD is a virtual device driver for a printer; and so on.