Are you talking about a "Wireless Network Adapter" or a "wired" ethernet card in the PC?
A "wired" ethernet card should be transferring data between computer and router at 100Mbps, so that wouldn't restrict the speed of your 30Mbps broadband.
An 802.11g Wireless Network Adapter has a nominal speed of 54Mbps but in practice is likely to be closer to 30Mbps. This could limit broadband performance, but shouldn't reduce it to only 10Mbps. With an 802.11n router and 802.11n adapter, you should have a wireless connection speed, depending on hardware, of 150Mbps or 300Mbps. Is the Mac perhaps running an 802.11n wireless connection?