Here's a link to where the latest available BIOS and drivers can be downloaded for this board: Downloads for Intel® Desktop Board D945GCNL. It does not appear that support is provided for Windows 8.1. Have you tried installing it already? Windows Update should (touch wood) have drivers to support your board. If you need a Windows 8.1 driver for the LAN interface before you will have access to Windows Update, you should check the RealTek site to see if they offer Windows 8.1 support for their RTL8111B-GR LAN solution. If they don't, you will likely be out of luck...
Hope this helps,