It's might be possible that the PCI adapter you're using to be limited to only 10 mbps. You can check the specs by typing its full model name on Google. Additionally, go to Network and Sharing Center then on the left Change Adapter settings. Right click the connection and select Status. You will see the current connection speed. If it display 10mbps, then that's the maximum supported.
Visit a local PC store to buy a new network adapter. They're very cheap nowadays, somewhere between 5-10 $.