Well, you are probably right about it being a settings thing. But just in case, can you desktop connect to the router? You should be able to start your browser and whatever your home page is, simply click on STOP and then type in 192.168.0.1 (this is typical, you might want to confirm with your manual). This should start your interface with the router. If it doesn't, then your problem is in the settings for your computer or your router. First thing to check is if the settings for both are identical, if they aren't, it won't work. Also, XP has an automatic Firewall that engages if you try to connect to any unsecured network. It considers a wireless network without WEP engaged to be unsecured. So you have to enable WEP. Question, is the desktop connected to the router by cable or by wireless? If its by cable, then you shouldn't have that problem.
If you can't connect to your router via wireless (if that's what you want to do) then just connect to it with a cable. Then it should work no matter what, but be sure you are using the right kind of cable. Once you get the desktop talking to the router, then its just a matter of settings. How your communications are set up and how your browser is set up. I always find it useful to start at the begining and go through the steps one by one. If you can connect via cable but not wirelessly, then you know where the problem lies. Its mostly a matter of elimination. Check this, if it works, you can eliminate that possibility and go on to the next. Since your laptop is working, you should be able to compare the settings on it to the settings on your desktop. That makes it really easy. Hope this helps. Let us know what works.