Does the same thing happen if you connect with another wireless device?
What if you do a hard reset on your router? This will erase all info, but easy to set up again.
Did you try using another Ethernet port on the router for the wired workstation?
I have a WRT54-GL, and have dd-wrt flashed on it. Works great, but I doubt that's the problem.
Yeah, let's do a hard reset on the router, turn off/on your modem, reinstall network driver
per FerchogtX on both PCs, and see what that does for you.