The factory reset did not work on my K9C3, so I used the USB image in the download section.
I used a USB stick formatted at FAT32 with partition name WINPE, and copied all the files from the archive on it.
Then I had to delete all the partitions on the tablet, because otherwise the script was confused. (Using diskpart, but there must be easier ways.)
It would not boot from a USB HUB, so I set the BIOS to boot from USB by default, and connected the USB stick directly (without a keyboard). That worked fine.
However, the image contains two or three viruses – run malwarebytes to remove them. Next I updated to Windows 10 1511 without keeping my files, so I am pretty sure I now have a clean installation.