I'm going to guess that both machines are using Windows XP. Firstly, make sure that you're both on the same "workgroup" (default is MSHOME). Then, simply navigate to the ITunes folder in My Computer on the computer, right click the itunes folder and press properties->sharing->share this folder. You can specify a name here if you like.
Now, on the laptop, go to "my network places" and wait a few seconds. You might have to press F5 to refresh the folders. Your shared folder shuold appear there, on your laptop, giving you access to the folder on your main computer.
If that worked: Go to iTunes, click file->add folder to library, and on the dialogue thats there now, find "My network places" (will probably have to scroll down), then click the + next to it, then you should have the folder you shared on the last paragraph. Click the shared folder, and press "ok".
There you have it! Your main computers iTunes stuff on your laptop! It might take a while to add to the library, though..!