If the PCs both have network cards in them already, and all you simply want to do is to connect the two PCs together (eg to copy files from one to the other), then your cheapest option is to get a Cat 5 crossover cable, plug one end in the one PC and the other end in the other PC - no need for a router and definitely no need for a switch.
Assuming you're using Windows, you will need to make sure they are both in the same workgroup, and not on different subnet masks. There's a useful thread about that... will search for it using the Search option on the forum.
by the way
