Be very careful what you do. You cannot connect a conventional USB cable between the ports on two PCs as to do so can cause damage. There are special file transfer cables designed for the purpose click here
A conventional ethernet cable is a much less expensive option. Is there a particular reason for not using this? Even if you have to install a network card in one of the computers, this would be the preferred method of "networking".
If both computers have Firewire ports (1394 Connections) you can "network" them for file transfers by plugging a Firewire cable between them.