at this place you can find a very complete manual +-1500 pages if you want to go through all that + some case studies etc...
i have set up a file and print server in the office not so long ago and i can recomend it above the xp pro as a server we had before.
to tell you the truth, i did print the whole manual but never actually used it, on most distro's the default settings are usually already close to what you will eventually need, so you will need to do some fine tuning.
in my case it was setting up the permissions on the shares correctly that took most work, so if you do it yourselve (something that can easily be done, i would like to suggest that you set up a spare box (this can be a very old slow dust collecting box)
and try too fool around on that one until you get everything right and get used too some basic linux administration.
just too make sure that nobody in the office will actually notice when you made the switch (
this is important because people will start too scream if it will not work for 10 minutes of precious productivity time)
with company's in the PA area i can not help you i'm afraid, for all other question i can recomend LQ.