Plug laptop and pc to the router using the cables. Use the network settings automatic for both systems. Check the ips of each system: Winxp double click the network icon close to the clock and System Preferences -> Network for macosx. Then open a terminal in macosx and type 'ping ip.of.the.pc'. Does it work? If yes, then you can start sharing folders. For winxp simply right click the desired folder and press share. Since your local network is protected through the router, you don't need to use any password protection (if no one else has access to your router). Sharing files on macosx is as easy: open System Preferences -> sharing -> turn on: file sharing and windows sharing.
Now your mac should show up in the network environment of the winxp and using Apple+K in the Finder of macosx and then typing smb://ip.of.your.pc will allow you to directly connect to the pc. To speed up the connections, you might want to make alias so you can simply double click the file and have access to the disk of the other pc.
After this whole thing works, settup the wireless lan connection of your powerbook and the router.
Good luck!
