damn_registrars's Journal: What is the cheapest / easiest way to setup WIndows AD?

(I submitted this to the front page as well, but since I am 0 for 20 or so I don't expect it to get anywhere)

I have been asked to help modernize a network for a small business. Currently they have 3 PCs running various versions of Windows and of course they have no central authentication for their logins or credential-based file sharing between the systems. One task I want to accomplish then is to set up a single point of authentication for all the systems, partially to better manage file permissions and partially so that it will be easier to handle requests for new passwords if someone forgets their login.

Of the three existing PCs two run Windows 7 and one runs XP Pro. We plan to replace the XP Pro system with a PC running 7 or 8 in the very near future. I expect that regardless, I will be setting up another PC in their network to handle authentication. I understand the old Windows SFU has been replaced with SUA in newer versions; if the new PC is running *nix could I have SUA sync passwords with it and be done? I know that the newest version of SAMBA also can act as an AD server, which is an interesting option as well although I don't have experience with that and I won't have a lot of time to set this up.

It seems like the nuclear option would be to buy a system running windows server, which should "just work". Of course that would also be the most expensive and I haven't run a windows server since NT4 so that would be new territory for me as well.