Slow WinXP Network

I have a little network in my home, composed by 3 PCs and 8 ports Hub: two running Windows XP Pro and one running Win XP Home Ed. All 3 PC access directly to Intenet with ADSL connection (dont know how to stablish one PC like Server and the others two like clients). Two of this computers have a directly connection between then with a crossover cable, but the two little monitors of a network connection in Systray sometime show 100 speed connection and other time show 10 MBps speed connection.
Why?

and

What can I do to make the connection speed of the PCs using a crossover cable always was a 100?

Other question: Where I can find a guide to make one of this PC act like Server and the others two like clients?

Thank a lott for your answer and time.