>We know that a VxD file contain driver for hardware on windows. But what exaclty is as vxd.
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It's a virtual device driver; the mechanism that Windows uses to interface to the hardware and publish a set of services back to the Windows environment.

>Is it like a DLL which containt fonctions that we can use to talk to the hardware, does it have to be register or you can copy it without problem, etc.
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It's role is similar to that of a .DLL, except that it runs in ring 0, and publishes services to the OS. Windows has to be told to load it as appropriate for the system; just copying it onto the system doesn't accomplish getting the OS to use it.

>I am asking the question because we have a VxD file that as grown from 900k to 2.2M. We think may be a virus did that but we'd like to know more about VxDs.
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The OS sometimes bundles a number of VxDs together at times to speed the load process and provide the current configuration data during system startup through a single, known file. What VxD is "growing"?