Well, you also had CA before ZA ...
Are you getting any of the errors listed in this KB: http://support.microsoft.com/kb/840634
If memory serves, the XP firewall is designed to block File and Printer Sharing from any network and allow access only in the same subnet in its default configuration.
To allow file and print sharing on your computers from any network, follow the steps described below:
1. Click on Start --> Settings --> Control Panel. If you are using the Windows XP Start Menu, you can find the Control Panel on the right side of the start menu.
2. In the category view, click on the Security center and then on Windows Firewall to open the configuration window. In the Classic view, you will see the Windows Firewall icon.
3. Check that the "Do not Allow Exceptions" box is not checked on the General tab
4. Click on Exceptions tab and highlight File and Printer Sharing, and click on Edit to modify the settings. The file and printer sharing is done through port number 445.
5. Highlight the TCP 445 and click on Change Scope (or double click) to edit it. In the resulting screen, select whether you want to enable for any network, for just your network, or for a custom list of networks. In case you choose custom list of networks, you will have to manually enter each subnet you want to enable for it to work. I'd suggest you NOT select ANY network as anyone who can reach your network will be able to access the computer over the network.
6. Click OK until you get out ... does this solve the problem?