Problem with creating home network with PC and Mac

Hi,

I have an I-book, a PC (running on windows XP) with a wireless PCI
card, a ADSL modem and airport express (acts as my wireless router). I
am trying to create a home network using these resources. My setup is
as follows: The ADSL modem is connected to the airport express and
connected wirelessly to the i-book and PC (via wireless PCI card). I
have managed to setup the home network successfully and able to surf
internet using the i-book and PC. However, when I turn off the
computers and switch on again on another day, both computers are not
able to surf internet. However there are still wireless connection as I
use Itunes to pump songs from both computers to the remote speakers. I
have already enabled internet sharing for I-book. I believe that it has
something to o with the configuration or settings of the ADSL modem.
Would appreciate if someone can advise how to set up a home network
with I-book and PC using airport express successfully. Thank you and
best regards.