Well this sounds a bit more familiar now, I could throw a guess and say the bootimg is corrupt. You may be able too fix this though I advise using Ubuntu to do it because of the lack of driver issues and a better response time. If you run something like
fastboot -I 0x1949 flash boot boot.img
with the kindle unplugged, and plug in the fastboot cable when it says waiting for device, then it should attempt to flash the boot image again, and even if it fails it might give a better result, because I have seen what happens if a bad kernel flash happens before, usually it will kick the device straight into fastboot mode, so in your case it could be beneficial neither way possibly.
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