I can't think why the adapter would negotiate a 100 connection unless maybe you are using an old piece of ethernet cable. Try using Cat5e or better. Also assuming you've installed the Realtek LAN driver available for download from the intel website.
right click and get properties for the adapter
under the advanced tab there's a property setting called Speed & Duplex. By default the value is set to Auto Negotiation. Set the value to 1.0 Gbps Full Duplex.
Hopefully that fixes your problem.