This is best posted in the Networking forum, but no matter;
We'll need more information (e.g brand and model of Router, how your PCs are connected to the router, all wirelessly or mixed wired and wireless, which security measure had you set ~ e.g. what SSID, MAC address filtering on, WEP or WPA, "Ping" response disabled, etc.).
You say you have setup your wireless network.
While connected via an Ethernet cable to your router and accessing its configuration page, using a browser (such as Internet Explorer and entering an IP address such as 192.168.1.1 or 192.168.2.1);
Did you change the default SSID to a name of your own choosing so you know its definitely your router your wireless Pcs are connecting to?
Did you add each MAC address for each of your PC's wireless components to the MAC Address filtering section? (You get your PCs' MAC addresses by Start> Run> cmd> "Ipconfig /all" without the quotes, and noting down the "physical address" which is the MAC address.
If its a second hand router, the settings you see are probably left over from the previous owner. You need to do some housekeeping and remove those which do not relate to your equipment.
Also to change the Router's configuration page's login password, the SSID name, WEP/WPA keyword/password.
Hope that is a good start for you.