If you're running Windows, you can do it straight from the "Network Connections" page by sharing the ethernet connection and bridging the two (wireless and ethernet). I have never tried it, but if you search on Google there are many instructions you can follow.
http://lifehacker.com/5369381/turn-your-windows-7-pc-into-a-wireless-hotspot
You can also try software called Connectify.
However, that being said, you should look into getting a wireless router, because it makes things much more simple (and your desktop doesn't have to be turned on to use the WiFi).