The Dell L series is a bit older than 5 years. My guess is about
1998-1999. Intel Coppermine PIII with 810 series chipset.The SB5100 has an ethernet connector:
Well, basically it's time to dump the USB. You will need to purchase a PCI ethernet 10/100baseT ethernet card and install it in your computah. These are about $15 and come with drivers. Your L series laptop probably came with Windoze 98SE. Be sure the card you buy has drivers for whatever operating system you're using. Many of the current models only come with W2K and XP drivers.
Once installed, the WAN (internet) port of the WRT54G goes to the ethernet port of the SB5100. Remove the USB cable from the SB5100 as it will no longer be needed or used. Another ethernet cable goes between one of the LAN ports on the WRT54G to your new ethernet card in the Dell. Do all your configuration with a wired connection and NOT via wireless.
You should be able to just plug everything together, configure the WAN connections page for whatever your ISP expects (probably DHCP), and be connected to the internet.
The rest is following the instructions for setting up the WRT54G wireless. Be sure to use WPA-PSK encryption for best security.