After getting the above error and as I could not find a repair of the same, I downloaded the latest Win10 and installed it. The pen drives (2) worked fully for the first 2 days But after just two days, it is again showing the same error.
Why pay for a key? That money is not going to go to MS for sure. Plus lots of fakes running around.
Look up with MDL forums, the old upgrade trick is still working. You will anyways need a format IMO.
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Optionally, why dont you move to Linux. Unless you need any specific WIndows software, Linux is a far better bet.
Try to pickup a RPi 2GB if you dont want to mess around, and then boot Raspbian.