Make sure that the DNS and default gateway are the same as your router. This is a necessity for the home LAN. i.e. 192.168.1.1 or 192.168.1.0. These options are set under Start -> Search -> "View Network Connections" for your devices. Open a command prompt (as administrator)
ipconfig /all
Check these machines and make sure they all have the same subnet mask, default gateway, and DNS settings. Remember, the DNS should be pointing towards the router.
Under the command prompt, type:
net view
From one or all of the computers. You should see the other systems.
You can try to access one of the systems using \\computer_name in explorer, over the network.
Try turning off simple file sharing on the XP systems especially, but also on the Windows 7 systems. This is accomplished by opening Explorer, hitting the Alt tab, going to Tools -> Folder Options -> View -> Advanced Settings
Make sure network discovery is on for all systems, including the XP systems.
Try disabling the software firewalls on the systems temporarily.
Try disabling the hardware firewall temporarily.
Please let me know if any of these solutions work and report back. Also see:
Windows 7 Can't see workgroup PCs