[String]$Username = 'customadmin'
[String]$Password = 'RakksFr0mR3DD1T'

$UsernameQuery = Get-WmiObject Win32_UserAccount -Filter "Name = '$Username'"
$BuiltinQuery = Get-WmiObject Win32_UserAccount -Filter "SID like 'S-1-5-21-%-500'"

# $Username exists.
if ($UsernameQuery -ne $null){

    # Check if $Username is builtin account.

        # It is the built-in administrator account. Configuring it.
    if ($BuiltinQuery.Name -eq $Username){

        Net User $Username $Password
        Net User $Username /Active:Yes

        }

        # It isn't the built-in administrator account. Delete it then configure builtin account.
    elseif ($BuiltinQuery.Name -ne $Username){

        Net User $Username /Delete
        $BuiltinQuery.Rename($Username)
        Net User $Username $Password
        Net User $Username /Active:Yes

        }

}

# $Username doesn't exist.
elseif ($UsernameQuery -eq $null){

    # Configure builtin administrator account.

    $BuiltinQuery.Rename($Username)
    Net User $Username $Password
    Net User $Username /Active:Yes

}