Where can't you reach these speeds?
1. Between connected PC's via the Router?
2. Between the Internet/WAN and your PC?
If 1, do both PC's have a Gigabit NIC/Capabilities? How do you measure the speeds? Do all connected PC's show a connection speed of 1Gb? Do you have a 1 Gigabit capable switch between your router and PC?
If 2, what is your internet connection speed and your modem type?
Cause 15MB/s sound like a 100Megabit connection (should be around 12.5 MB/s).. so maybe your one of other(!) your connected PC's or Modem has a max 100Megabit network connection. So devices connected at endpoints of the LAN and WAN have to be 1 Gigabit to get 1 Gigabit speeds between those devices. If you have a 100 Megabit max modem, you internet won't get faster than 15MB/s. If you have a three PC's, two with Gigabit connections and one with a 100Megabit connection, only transfering information between the PC's with the Gigabit connections will get you speeds above 15MB/s.
If above things don't apply to you, check if your connection speed indicates 1 Gigabit in windows, it COULD be a driver problem otherwise, but usually the 8111C network chip is supported perfectly in all windows versions. Go here for drivers: http://www.realtek.com.tw/downloads/downloadsView.aspx?Langid=1&PNid=13&PFid=5&Level=5&Conn=4&DownTypeID=3&GetDown=false