Not sure where you are in the setup so I'll start at the beginning.
I assume that the printer is attached to the main computer via a USB cable. In this case the main computer must be on, along with the printer. On the main computer, go to the Printer Properties. Click on the Sharing Tab, select 'Share this Printer' and give it a name. I'll refer to the printer as 'printer1' to be original. For the next part you'll also need to know the name that you gave to the main computer. This is found in Control Panel > System > Computer Name, you want the one listed under 'Full Computer Name.'
Now, on the laptop, go to the wizard to 'Add a Printer.' Click Next, and then select Network Printer rather than Local. On the next page, select the second option 'Connect to this printer ...' It shows that the name needs to be entered like '\\server\printer.' Replace 'server' with the 'Full Computer Name' from above, and replace 'printer' with the name you gave the printer earlier. If necessary, you may need to install drivers for the printer.
If this fails then the chances are the problem is related to the network rather than speocifically to the printer. A few things that would be helpful to know are:
Can you share files over the network ?
Are there any other firewalls other than the XP ones ?