GREAT, Brink! It finally worked. I read that thread at Technet and downloaded PsTools.
First I tried the batch file to avoid difficulties, as recommended on a post about the middle of that thread, but it gave me an error:
Couldn't install PSEXESVC service:
Access is denied.
Then, I tried the normal direct way on an elevated command prompt and Regedit opened righy away. I didn't have to use permissions and could delete that key directly from its context menu. It couldn't be easier. From what I read it would not be possible to change any security setting or delete the key with that Owner Rights entry. It was the same problem the thread starter had, even if with another key.
Thank you very much for your great and helpful advices.