I have read some where by which we can convert our old cpu in to server.Is this really possible ? well it will be useul for me if u can explain how to convert it ?
What exactly do you want the server to do? If it's just a file repository, look at NAS or just enable networking and sharing on it and remove as many applications as possible. If you are trying to host a web site, FTP or email mailboxes, look at https://www.pablosoftwaresolutions.com/html/products.html If you want a video or media server, look at Serviio.
Otherwise, if you want to run aplications remotely on it, you could consider one of the many Linux based servers. Ubuntu Server is probably easiest to use.