Windows 7: Struggling to use my laptop as an adaptor

Hey all,
the kind people on this site helped me once before, when my computer was going mental with blue screens. Now got a small problem, something I haven't been able to find a solution for yet.
Basically, I moved to university last year and am in a building with a wireless router. I brought my desktop PC and also my laptop, as I enjoy PC gaming when I have some spare time. Now I don't own a wireless adaptor for my xbox360 or my desktop pc. So I was told I could use my laptop as a wireless adaptor for either the desktop or the xbox when i wanted to do something online. It worked flawlessly for the 360, but having problems with the desktop. I have connected it up via an ethernet cable and enabled sharing on the network but the desktop still can't connect to the internet. It says something about the local area connection not having a valid ip address, but I've searched and could not find a solution. Bare in mind I'm not too good technically with computers, so keep it simple if you can.



Cheers in advance for your help.