First you have to set up internet connection sharing on the laptop.http://www.youtube.com/watch?v=ifDyMaWC0Mk
(I just grabbed a random tutorial off of google, feel free to find a better one if that tutorial doesn't help)
After you have that set up, your going to want to run an ethernet from the laptop's ethernet port to the input/modem port on the router.
Then plug the desktop into one of the output ports on the router.
This assumes you are sharing the laptop's wireless connection over ethernet.