|If you can't ping it is most likely that a firewall on the PCs is preventing communication.|
Are you using a third part firewall such as Zone Alarm or simply the included Windows Firewall?
What ever you have I would check that there isn't a rule in place preventing the PCs from seeing one and other.
How are IP addresses allocated to the PCs?
Are you using DHCP from the wireless router?
In order for the PCs to be able to see one and other they have to be on the same IP range and subnet.
The fact that they both connect to the net okay implys that it's unlikely to be an IP address issue.
Check your firewall first.
IT Services Manager