Good that you are listening.
I presume that you have in the Internet protocol (TCP/IP) properties of your Local Area connection in the Desktop, have ordered it to obtain the IP addresses automatically. In that case you would have turned the DHCP server functionality in your modem/router on.You can in the LAN settings of your modem restrict the DHCP server to assign IP addresses upto 192.168.1.3 instead of any upto 192.168.0.253. That way your Desktop and Laptop won't get anything other than 192.168.1.2/3.So include from 1 to 3 in your Zonealarm's trusted IP addresses.Zone alarm>firewall>zone>add.
Instead of adding individulal IP addresses you can also add them in the range say 192.168.1.1 to 192.168.1.3. That is all to it.
If you do not want to enter into your modem and change the settings simply add the range 192.168.1.1 to 192.168.1.253 to the Zonealarm's trusted zone.I will however not recommend this for security reasons.
In my wireless network, I have further linked the MAC address of each PC to the IP address that can be assigned to it by the modem/router/ access point. That way my desktop always gets 192.168.0.2, my Vista Lappy 192.168.0.3 and my Mediacenter PC 192.168.0.4.This apart from the WPA2 security.So no outsider even within the wireless range can draw juice from my internet connection/PCs. As in your case my Desktop (XP) is the main PC directly connected to the modem/router and protected by Zonealarm. And make sure to disable the windows firewall. Hope I have made things clear to you. Good luck.