Networking wired and wireless computers

I have a laptop connected to a dlink router that works absolutely fine. i also have a desktop connected to the same router by wire, also works fine. both on windows xp

i want to know how to connect the 2 computers together so i can share files between them. the windows xp wizards dont seem to cover this case.

I have a usb dongle for the desktop but am having a horrible time installing so would prefer to do it wired/wireless

thanks