Built for Rock'n'Roll (i was)

A. Acudiendo al registro del sistema: HKEY _ LOCAL _ MACHINE / SOFTWARE / Microsoft / Windows NT / CurrentVersion / SoftwareProtectionPlatform y en lista de variables buscamos "BackupProductKeyDefault". Ahí debería estar.

B. Ejecutando PowerShell, como bien resaltan en los comentarios, introduciendo lo siguiente: powershell "(Get-WmiObject -query ‘select * from SoftwareLicensingService’).OA3xOriginalProductKey".
(*) Esta segunda opción a mí no me ha funcionado.



In PowerShell, you can directly access the raw licensing data like this:

PS> Get-WmiObject SoftwareLicensingService

You can also check the license status of your copy of Windows:

PS> Get-WmiObject SoftwareLicensingProduct | Select-Object -Property Description, LicenseStatus | Out-GridView

And you can find out which Windows SKU you are actually using:

PS> Get-WmiObject SoftwareLicensingProduct | Where-Object { $_.LicenseStatus -eq 1 } | Select-Object -ExpandProperty Description
Windows Operating System - Windows(R) 7, RETAIL channel

To investigate all the other logic found in slmgr, have a look at the VBScript source code:

PS> notepad (Get-Command slmgr).Path