The hidden windows built-in account can be seen when you boot to SAFE MODE. People usually skip the setup of this account so it usually doesn't have a password. If it has one, first try to just press ENTER with no password. If that doesn't work then try using "password" or "admin".
Windows always requires to have at least 1 admin account. If you are an admin account then it is possible to delete the builtin admin account. If you are not an admin account then it won't let you delete the default admint account. (you'll need to change your account into an admin account first.
also read this:: 