I'll bet you're already properly connected physically. That 4-port ADSL router is a router and switch. So all you need to do is tell EACH computer to share files and printers. On OS X it's under Sharing Preferences. On XP, it's done item-by-item; if you want to share a folder, you right-click on it in Explorer, and then click "Sharing" and follow the steps. You may want to use the wizard to do this for you if you're unsure about the steps XP needs. Sharing a printer is simple too-open Printers and Faxes, right click on the printer you want to share, and select Sharing.
Note that the varying lengths of ethernet cable you have currently won't make any difference in performance, because the standard allows up to 100 METERS of cable between devices. Bet you aren't going that distance yet!