this is a big task.
first you have to override the Windows login, that is no that easy (but doable, I remember reading something about changing the standard login hacking the computer registry)
second, you need sofware to compare two images (in this case two faces), and that is the big problem that for now still does not have a clear solution (what if I grow a beard, if I put colored contacts, if I have a hat, or if I just put a picture of my boss in front of the camera?) You can make big bucks if you write it! But google for "compare faces" or something like that, maybe you find something suitable just for your needs.
And anyway, VB is not the language for this task.
"There are two ways to write error-free programs. Only the third one works."