OH! You want it the other way around? That's a different story. There are a couple different options here. First one is you can bridge the wireless and wired connections and connect the laptop to the desktop with a crossover Ethernet cable, CAT 5e if I'm not mistaken..
Or you could buy a wireless card that goes into a usb port and put it on the desktop. Or get one that goes in a free PCI slot inside the computer on the motherboard.