This is almost certainly a firewall problem. The personal firewalls will generally not allow you to receive a ping, so it fails. Try to ping each of these from the gateway. Try putting both on the wired LAN and repeat the test PING between the laptop and PC and the gateway and both devices.
Does that work? If yes, then the firewall is on the wireless router, and is preventing traffic between the wired and wireless networks. This is often just a check box in the config to allow this, but be aware that early versions of XP bridge the wired and wireless networks, so allowing this on the wireless router can cause a loop (usually no spanning tree on the wireless routers).
If that didn’t work, then it is the personal firewalls on the PC and laptop, no question about this, that WILL be the problem.
If the PC on each side can ping the default gateway but the pings do not go across the WAN, then it is possible that the subnet mask is wrong on the end client computers. This is a typical symptom if the clients have no problems communicating on local LAN but have trouble across a WAN.