So, I've figured out why my laptop showed as, "Already registered":
The registration process took place during the Windows activation, with my Microsoft email account (in this case, an old Hotmail account that I do not use as often as my Gmail account). Whatever registration message that got there probably was deleted long ago in the spam folder, but I could recover the account using the email (which I did). The problem is that I now have an Acer account with my Gmail account (with my community posts and a Tinkerer ranking) and the Acer account that have the register of my laptop, with my Hotmail account (this one).
Is there a way to merge my old community profile, under Gmail, with my Acer account, that has the laptop registered (the one with Hotmail)?