Problems with LAN

We recently installed a LAN card in our computer.
The problem was discovered when my son and his friend hooked their computers up to each other and my son could not see his friends data files making it impossible to play a LAN game.
My sons friendhas tried our card out in his computer and played the game with other people so the problem appears to be with our computer or how we installed the Lan Card or drivers.
Both my son and his friend have XP pro, 1000 mghz, 156 ram, 20 gig hard drive.Our computer is also stating that the LAN card is installed and functioning.
I hope somebody has some tips for us.
Greatly Appreciated
Mirralee.