So how are you connecting these pc's together??? I will assume here that the Desktop PC is the main one (always connected to the net) and that the Laptop is the one you are wanting to connect to the Internet, through the desktop one.
I have done this before, and it's quite simple to set up, however one restrictin here is that for you to get online, the main desktop will have to be on. Numerous times my parents switched off our main comp at home when I was playing online with mine, after forgetting to check with me first.
What you need to do is the following, assuming Windows XP is set up on both of them.
On the Desktop, you will need to have two ethernet cards (We shall call these card A and card B). Card A will be connected to the internet, and card B will be connected to the Laptop via a straight through network cable. Next, on the desktop you need to run the network setup wizard, located in start all programs -> accessories -> communications. In it, select the option (normally the first one) that is labelled something like "This computer is connected directly to the internet. The other computers on my network connect through this computer."
After this, follow the prompts, selecting which one of your LAN connections is the internet connection, and selecting which ones are connected to other computers. Knowing Windows, you may need to restart here.
After this step is complete, go into the network connections folder (from control panel) and make sure that Internet Connection Sharing is enabled on the LAN connection. If it is, you will see a hand underneath the icon. If not, right click your internet connection, select properties and navigate to the advanced tab. Make sure that the checkbox for Internet Connection Sharing on that connection is enabled.
Next, run the network setup wizard on the laptop. You need to select the other option here, the one that says something like "This computer connects to the internet through a residential gateway or other computer on my network"
Follow the prompts and let it do its stuff.
Then go to the connection on the laptop that is connected to the main pc and go to properties. Click on TCP/IP in the top box, and select Properties. Make sure that "Obtain IP address automatically" is selected, as well as the other option there to obtain DNS servers automatically.
Then, on the laptop, go the the network connections section. You should see a new globe-looking icon that is called something like "Gateway on X" where X is the name of the main desktop computer.
If it's there, you are online!
Find me on MSN or XFire if it doesnt work or you need further help.