it is but i've posted many times on here about it and havent had a great deal of luck with responses - think that not too many people have tried it.
Do a search on here for active directory and you'll probably find a few posts on the matter. But just in case you dont already know, the tools that you're after are called LDAP (lightweight directory access protocol i think), and there's a dll that you need to put in the php.ini to get it to work - i couldnt find it anywhere, until some nice bloke posted it for me (and now i do the same for you).