Unfortunately, you have to configure Windows with a program called PGina.
Mac OS X 10.7 and later doesn't support SMB authentication anymore since Microsoft has ditched it's support of the authentication portion of the SMB service. (windows 7 authenticates differently now)
With that, that means that 10.7 server will not accept authentication requests from Windows Boxes right out of the box.
PGina is a piece of software that allows Windows to authenticate against the Open Directory - directly.
It works, but it's very rudimentary. You can't run scripts on logon from the server. You can't have your windows compy's directly connect to a "Home Folder" automatically. The windows compy will create a local home folder for the user upon logon, and leave you with no automatic tools to sync back to the mac server.
Essentially, PGina really is just a starting point. If you're a developer, or a keen IT manager, you'll find this exciting, but if you're a novice user that has need of a server system you'll find it a bit useless.
I hope this isn't a game changer for your OS X server... It's a great system that's recently been updated and thinned out. Almost like they're considering a rudimentary rewrite of a lot of the systems. I'm sure that given time, Apple will bring back a lot of this functionality, but for the most part if you need to have these features people are sticking with 10.6 server. If your server didn't come with 10.7, you might consider getting your hands on an older version.
Hope the advice is worthwhile for you!