Emulating Domain Logon

First off... I'd like to say that this post could probably fit into about five different forums, so I just picked the one that would probably get the most traffic and benefit the most people if anyone has a solution for me.

My company puts in a lot of Windows XP Professional computers and networks them together for small businesses. The biggest problem we have is situations where the companies are to the point of sharing lots of files, but not up to the point of purchasing a full server. Normally this isn't a problem with two or three PC's, but when you have ten, you have to make sure that anyone sharing files also has a username/password on their local PC for everyone that will need access to the files or printers on their computer.

That said... is there a simple way that I can emulate a Windows NT/2000 domain without actually having a server. I just want a way to authenticate users to one computer and avoid putting ten usernames/passwords into ten computers. I know there are "network appliances" out there that will do this, but this is not an option for me... money is an issue since these are small businesses that are watching every penny they spend in a slow economy. I'm sure there is some software out there that will do this, I just haven't found it yet. The less effort I have to put into making this work for my clients, the better. Any ideas?

dont know if this will help, you can take a look
http://www.tekbabble.net/nt/serveremulate.html

Cool utility... but not what i'm trying to do.

I'm trying to, for lack of a better term, fake a Domain Controller. I have networks with three to fifteen computers, but no server. If users want to share files, you have to go PC to PC and add usernames/passwords to all PC's that will be sharing files/printers. I could just autologin everyone with the same username/password, but that is a security problem (employees do not need to have access to each others e-mail, personal work files, or information like accounting info.).

Is there some kind of software that will make a 2000/XP Professional emulate an NT/2K domain? I know I can do what I want with a Linux box running Samba or something like a Cobalt Qube server appliance, but that would require extra cost for my customer... which is not an option. Someone else has to have wanted to do this. Is there something/anything out there that will do this????