Go here and download the aboot, boot and laf image files:
Make sure to left click on those and wait for the popup to save. Once those three are saved to your machine, move them into the sdk-tools folder on your C drive.
Then while the phone is in fastboot mode and plugged into the PC, again navigate to the sdk-tools folder via command prompt and then flash the files to the phone. So the commands will be...
cd c:\sdk-tools (if not already there, otherwise you can skip that)
then...
fastboot flash aboot aboot.img
fastboot flash boot boot.img
fastboot flash laf laf.img
Wait for each of them to flash before moving to the next command. Once done, disconnect the phone from the cable. Also close your windows command prompt at this time.
Now long press power button for NO LESS than 15 seconds to completely power off the phone. Once it's off, press and hold volume up while at the same time plugging in the usb cable from the PC. You should now get download mode. Let us know if that worked, then we can proceed to flash the stock firmware.