Are you simply trying to access files from one machine to the other, or do something else?
By default, within XP, without a domain controller, you would not (typically) be able to "Browse" for computers through Network Neighborhood. Which it sounds as though perhaps you are attempting to do, and don't see the machine.
This can be done, but isn't advised as it requires the NetBUI / NetBIOS protocol to be installed and run, which again, isn't advised. TCP/IP should work fine.
So, seeing that you can ping from one machine to another, through the router, the router is passing the traffic from one to another. So, that's a good thing.
Depending on how you have ZoneAlarm configured (assuming you have tried this with it on, and or off) it will effect the problem with other computers having access to the machine, although will answer a ping request, but deny all others. However if the problem is the same with Z/A on or off, let's try something else.
Your XP machines should be DHCP as well if your router is DHCP, so you won't have any configuration for IP address, Subnet, etc. If you want everything in the same work-group, that's fine, confirm that they are.
Once you have checked that the machines are all DHCP, get their IP addresses and confirm the following:
Machine A/B/C IP configuration.
Should open a "DOS" window
This will give you your IP config, and most importantly, tell you / confirm that it is assigned by DHCP.
With each IP address, ping the other devices from that same DOS window.
If all machines can ping on another:
Confirm file sharing is enabled;
My computer > RightMouseClick on HDD, Sharing. Confirm sharing is enabled.
One that is complete, try the following:
From one computer click start>run> In the Run Box type \\192.169.X.X\C$
192.168.X.X being the IP address of the other machine you are trying to reach. C$ being the C drive. (or use D, E, F, etc. if you shared that drive with another drive letter. C$ will always get you to the C: drive of the machine.)
This should open another window with login User name / Password for you to enter. (if your machines are secured)
Try that and let me know what happens.
p.s. I know absolutly *NOTHING* about computers, but I did stay at a Holiday Inn Express last night.