hi,
Take a Gnutella-Client (e.g. LimeWire/Bearshare) and disable the autoconnect function. There's always a Textbox to enter manually a server-address to connect to. Type in the network-name of another computer connected to this LAN. .... you're connected. From now on you can share or download files...

note that the gnutella-client has to be run on every machine!

Pferdo