No Internet via LinkSys W/L Connection

I am assisting a friend in setting up his home wireless network.

He has a new Win XP SP2 desktop wired via Ethernet LAN to a LinkSys W/L
Router [WRT54GS]. We installed Win XP2 and a LinkSys PCI W/L Adaptor Card in
his old desktop.

We configured the wireless settings and pre-shared key for encryption. We
can see files from the other computer on both desktops - so file and printer
sharing works.

However, while we can access the Internet from the new desktop, we are
unable to access the Internet from the old desktop via the W/L Lan. The
Router is configured properly - at least as far as we can tell [we had full
Internet access from both desktops when they were both connected via Cat 5e
cable to the Router]. We have run through the Network troubleshooter a
couple of times without any luck.

Any suggestions would be appreciated!
--
Dave Fulton, Principal
FULCO | IT Services
Calgary AB CANADA