Can I network two PC's just by using a USB cable from one to the other?

There are special cables designed to do that, but a regular USB cable would not do it. As you may know, a USB cable is rectangular on one and and almost square on the other. Neither PC would have the square connector, and even if connected, they need the translator box that is inline on the special USB/USB data transfer cable.

IF both computers have an ethernet card, you can get a crossover cable that will allow them to connect.

You would then need to setup a network, assign both computers IP addresses(192.168.1.1 & 198.162.1.2), put both in the same workgroup, subnet mask (same on both machines 255.255.255.0), and enable file sharing.

Now that they are connected, what do you want to do?

Transfer files? Just drag & drop...

Share internet connection via broadband? You'll need a second ethernet port in one machine to connect to the cable/dsl modem.

Sharing via dialup? ...Not worth it...too slow...but if one machine has a modem, possible. Just enable ICS. (see Windows Help for your version)

Nope. You will need an ethernet 'crossover' cable, network card in each computer, and use ICS.