The Windows Firewall [installed in XP by Service Pack 2] is a big improvement over the old Internet Connection Firewall [ICF] that comes with XP out of the box. Let's say you have a home network and you want to allow one or more of the computers on the network to access files, but you don't want all the computers on the network to be able to do so. Setting user account permissions on the files and folders won't work if it's connections from specific computers, rather than specific users, that you want to block. However, you can use the Windows Firewall to do it. Here's how:

1. Click Start | Control Panel | Security Center.
2. Select Windows Firewall.
3. Click the Exceptions tab.
4. Check the box for File and Printer Sharing. At this point, by default all the computers on the subnet will be able to access the file shares.
5. With File and Printer Sharing highlighted, click the Edit button.
6. For the associated ports, click the Change Scope button.
7. Click the Custom List button.
8. Enter the IP address(es) of the computer(s) that you want to allow access.
9. Click OK.

Note that you can also use this feature to allow access to all computers, even those on the Internet (NOT recommended).