How do I do that ? ... as the only account I assumed (it seems wrongly) I was owner of HDD's
You are only the owner of the drive (technically only the directories and files) that you re-installed Windows on. The other existing files have the same rights they had before, which is the accounts on the old Windows installation.
The way Windows security works is that once you delete a user, even if you create a new one with the same name, it's NOT the same account and needs to be given rights again. Think of it that way. You bought a house with an existing locked room (house = Windows, locked room = your old drive). Even though you are the new owner of the house, you don't automatically have the key used on that locked room, you have to get it from the old owner or make a new key (take ownership of the files).