Scan for viruses and spyware. Use a personal firewall on each computer.
Download & install latest firmware for the WRT54G.
Run Windows Update and install SP2 (if running XP).
Download and install latest drivers for your wireless adapters.
If the adapter has newer firmware, update that, too.

What are the model numbers for the wireless adapters?

Do you have any wireless security enabled to keep neighbors off your network?

WRT54G broadcasting in what mode: b only, g only, mixed?

NOTE: Updating the firmware solves many problems for routers.