The most likely problem here is a firewall blocking access.
1. Windows has a firewall
2. The router will probably have a firewall
3. You may have other AV software with a firewall on the wireless PC.
Get these turned off to get the network set up.
Other things you could try would be to put a cable into the wireless computer to eliminate the wireless connection. I guess you have Mac filtering / WEP / WPA encryption on the router and this could also be causing a problem.