Please help – Password protecting files over a network?!

Hi everyone

I have a network of computers at a clients site with a desktop machine running XP pro as the server. The files sit on the server and everyone has a mapped drive to their own folders but at the moment can also get to each others files through network neighborhood.

Any thoughts? I need to stop people getting to each others files. Because they are not using a “server” as such they have no DC and the only help I can find is for machines on the same domain.
Help!

Thanks in advance