What do you want your "server" to do? If you just want a central place to store your files, follow Schm1tty's advice, but with a little tweak.
Rather than having to remember your computer IP addresses (and figure out how to assign a static one which carries other difficulties) I would setup a windows Workgroup. By default, Windows machines are on a workgroup (network), usually named something like "MS_HOME" or "WORKGROUP".
Here's a tutorial on how to setup your home network so that XP and Win 7 machines can interact: http://www.howtogeek.com/howto/windows-7/share-files-and-printers-between-windows-7-and-xp/
You can use this method to share printers too, which is good if you don't have a printer that attaches directly to your network.