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#21
You need to delete all partitions related to the Windows OS. This means you leave any data partitions. Hopefully you can distinguish the two by their name.
You may feel more confident with this using a partition manager boot disk, but that's more steps.
(I've just been thru this myself resolving an issue).
Note: if you follow our recommendations, you will have made a current disk image (e.g. Macrium Reflect (free) ) before doing this: that way, you always have a way back if you make a mistake.