Download the firmware and save to Desktop - https://s3-eu-west-1.amazonaws.com/router-firmware/original-firmware/WNDR3800-V18.104.22.168.bin
Plug an Ethernet cable in between the PC and any of the LAN ports on the router
Set your PC's ethernet card to use a static IP of 192.168.1.10, subnet mask of 255.255.255.0 and gateway of 192.168.1.1
Turn the router on and follow the instructions below
Install the following TFTP software - http://kin.klever.net/dist/pumpkin-2.7.3.exe
Open the Pumpkin TFTP application and click on "Put File". In "Local file" choose the firmware file you downloaded In the "Remote host" field enter 192.168.1.1 - then click OK.
You should now have some indication of the file being transferred. The firmware on your router will be restored and the device will automatically reboot once done.
This can take up to 5 minutes, after which you should reset your PC's network IP back to what it was before (DHCP etc).
The router should now provide you with a 192.168.1.x address and you can access the original Netgear router page at http://192.168.1.1