Other computer seems cannot access Internet or files. Wireless network help.

I want both of my computers to share the same Internet connection and be able to access each other's files at all times.

OK, upstairs I have the old computer. It is the one that is connected to my cable Internet connection with a Toshiba PCX1100U modem.The new computer is downstairs.

I have a Linksys WRT54G wireless broadband router (2.4Ghz, 54Mbps) upstairs with the old computer. The old computer also has a Linksys LNE100TX Fast Ethernet Adapter which is a normal network card.

The computer downstairs just has WMP54GS wireless network card (2.4Ghz, 54Mbps).

My problem is that the computer downstairs can't connect to the Internet and neither computer can access the other's files. Everytime I connect my router I can't access the Internet on both computers.

Anyone have any clues as to what could be happening? It has worked before I just have no clue how I did it.