my drama update:
After the laptop, I did the upgrade to Win10 for the desktop as well, the PC had Win 8.1 Pro 64 fully updated and signed with the same MS account, it was reserved for the Win 10 upgrade, and the reserved upgrade was ready to go, indeed this time I used the Windows Update instead of the Media Creation Tool.
I tried to replicate the same procedure I did with the laptop, so I changed the Computer name accordingly, for now I'm sticking with an easy scheme, es: ZORRO-LAPTOP and ZORRO-DESKTOP, just to have a bloody clue of what is happening in the MS account page.
My laptop is currently activated and I can see it enlisted on the devices list from the MS account page, it shows the Computer's name and the brand of the laptop, the OS version is there also; the Computer's Name is visible in black font color while the brand is a gray color.
The Upgrade to Win 10 went fine and the OS was activated, the device was added to the device's list in the MS account, es: ZORRO-DESKTOP Gigabyte; all seems fine then I did a clean install(format), I made a mistake to not disconnect the lan cable, so the installation used some upgrades before moving to the desktop.
I had created a local user during the installation and then switched to the MS account for the activation, the device's list in the MS account had 2 instances of ZORRO-DESKTOP Gigabyte after the clean install, and it didn't activate, 0xC004C003 error.
I removed the 2 device instances for ZORRO-DESKTOP and tried again with another clean install, this time without the lan cable, after switching to the MS account there was no sign of the ZORRO-DESKTOP on the account page, and still 0xC004C003 error.
I did a full reinstallation of Win 8 Pro 64, using he same schemas (ZORRO-DESKTOP), and I did all the updates up to Win 8.1 Pro, I was signed in the store of corse, and the apps were updates(One Note had a error but really....wtf). I did a checkdisk(GUI) of the system C: and a sfc /scannow, this time I made a disk backup stored on a secondary HDD. no cleanups or registry clean were made so I did an extremely 'raw' or 'linear' upgrade to Win 10.
Happened that the OS was upgraded, activated, but ...., in the MS account page i had the instance of the laptop and a new one called PC , which means as I esplained before, no Computer Name, no Brand .... just a bloody PC, and no version of the OS!!!!!! ooh dear this is a quest.
In the search of the Holy Grail, I made some PC reboot to see if the account get syncronized somehow, something changed, there was a next link right next the OS version in the system window(where you see PC specs and activation status), this link will ask you to change the activation code(I didn't continue), rebooting the PC made that link go away; anyway nothing changed on the device list in the MS account page.
I removed the device from the MS account and it asked me to make a Factory Reset to assign the device again to the account; so I did a Factory Reset maintaining the user stuff, I'm not reselling the PC, I'm just resetting it; the process completed and the system resetted correctly and activated as well ...cool; In the MS account I had the laptop device(correct) and 2 instances of the PC, ZORRO-DESKTOP Gigabyte (Windows 10 Pro and version) and again PC .....wtf! and the next link was back into the system window.(reboot didn't fix it this time)
That was still the upgrade version from the Win 8.1 Pro process, I didn't made any clean install, I want to be sure before moving on; I did a checkdisk(GUI), I did a sfc /scannow which reported some damaged files it couldn't repair, rechecked the updates from the Windows Update and the Store as well, then again sfc /scannow still giving some warnings on damaged files it couldn't repair.
I Disconnect the MS account in favour of the local user the I did a reboot, after the boot up completed and logged into the desktop I checked the system window and the next link went away(some updates went on in the background), Nothing changed on the MS account.
I switched back to the MS account on the PC, and the next link back in, a reboot removed the thing this time; nothing changed on the MS account.
I did a winver this time ....and, the version is showed up correctly but on the bottom of the windows it say the licence is in use for: microsoft microsoft, yes 2 times microsoft; so I did a check on the laptop and there i got the local user name; this reflect somehow what is happening on the MS account page for the device list.
Now what that microsoft in winver means? means the OS is granted to a MS accounts? so why the laptop has the local user name i made? the license maybe means its not granted to me at all?
the laptop has the grant to the local user, and I did 2 clean installs(format) and it was activated without problems(well actually there is a twin story on the laptop some posts early in this thread).
I didn't perform a clean install(format) on the PC yet, today its too bloody hot I need to wait for a clean install.
If you have the 0xC004C003 error please check your winver and your MS account devices to see if you have the same issue.
edit: also if you re activated check the winver on your PC and compare with your MSA(Microsoft Account) this could help some folks to understand if their system went upgraded correctly.
Looks like its recommended to wait some time after the upgrade to Win 10 before adventuring in a clean install, run all the upgrades you can, do a sfc /scannow could help, check your winver and your MSA, reboot sometime, just play with it; the hardware ID that Microsoft could store to align the hardware to the licence could take some time.