Cannot access modem from desktop but fine from laptop?

I was having problems with my desktop so I reloaded Windows XP onto it. In the meantime I have had my laptop connected directly into my cable modem to use the internet for a couple of weeks. I want to have my wireless network downstairs from the desktop and according to the router and cable company instructions I need to be able to access the internet from the computer (the dekstop) in order to install the router. My problem is that I have the modem all set up and connected to the cable and into the desktop but it won't connect to the internet. In this same configuration, if I plug the Ethernet to my laptop I have my internet connection. Because I just reloaded Windows XP and it is a completely clean slate, is there anything I have to do to that computer to allow it to go online because it can't be a problem with the modem since I am using it to type this question. What should I do? Thanks!