need advice.

I service multiple home based users on a “bartering” basis, servicing some systems for a gentleman I was paid with a “server” (dual core P3 450). Never having used or worked on one I have no idea what tools I should use or what they do. Also this came with reformated drives and no OS, (clean as a babys behind!!) Are there any good freeware systems that I should look at? and could you reccomend some referance sites that I can use to understand the unit better?