| >> so what we have to do to make it server? <<|
Nothing because it is no possible without third party software. Just because you may call it a server does not make it so. You just configure it the same way as all the other computers on the network are configured.
You cannot configure Windows XP or Windows 7 as a server. They can only act as a peer in a peer to peer network or as a client on a file server network. That requires a Network Operating System which Windows XP and Windows 7 are not.
All versions of windows since Windows 3.1 have peer to peer networking built in and all versions are capable of communicating with each other. All you have to do is read the manual as each does it slightly differently.