you want your laptop to act as a gateway so your desktop can get the internet through it?
or you have a dekstop which is connected to a modem and the router connected to that and u want the laptop to get the wireless signal?
depending on what situation you have, the solution is different.
I'm assuming you want to connect your desktop to the internet by using your laptop's wireless internet connection. More specifically, your laptop currently has a wireless internet connection, and you want to use a cable to connect your desktop to the laptop and share the wireless internet between the two, right?
First, make sure you have an ethernet port on your laptop where you can plug in an ethernet cable. Some laptops don't have them. I'm guessing you do since you want to connect the desktop to it.
What you want to do is share your connection, not bridge it.
You'll need a specific network cable called a "crossover cable". It's different from a regular ethernet cable in that it's used to connect two computers to each other, rather than connecting a computer to a modem/router/hub/etc. It looks the same, but it's wired differently on the inside. You can usually get one from an electronics store like Radio Shack, Best Buy, or even Staples and Office Depot. Just make sure it says "crossover" on the package.
On your laptop, you'll want to set up Internet Sharing. To do this in Windows XP, connect to the internet via your wireless connection like you normally would, then go to your Network Connections: My Computer -> Control Panel -> Network Connections.
Once there follow these steps:
1. Right click on your wireless network adapter, and go to properties.
2. Click on the "Advanced" tab, and put a check in the box where it says "Allow other network users to connect through this computer's Internet connection."
3. Click OK, and go back to the Networks Connections window. Look at your wireless connection again, and it should say "Connected, Shared" under it. This basically turns your laptop into a router.
4. Connect your desktop to the laptop with the crossover cable, and connect to the internet. It should work as long as your desktop is configured to automatically receive an IP address. Your laptop will give your desktop an IP address so you don't need to set up a static IP for it.
You should be good to go after that. I just recently used this method to share my wireless connection with my Xbox 360 instead of buying the pricey wireless adapter for it. Works great.
You can do this with Vista as well (you didn't mention if you had XP on both PCs) but the process is slightly different. Good luck!