While tools like ProduKey or Belarc Advisor are popular, you can do this natively via PowerShell (if PowerShell 2.0 or higher is installed):
There is that works on all installations. If a website offers a single key for everyone, it will be blocked by Microsoft’s activation servers (or fail offline activation checks). product key windows server 2008 r2 standard fix
This is the preferred method for fixing stubborn keys, especially on Server Core or Remote Desktop Servers. While tools like ProduKey or Belarc Advisor are
Troubleshooting Product Key Issues in Windows Server 2008 R2 Standard product key windows server 2008 r2 standard fix