Post date: Nov 29, 2012 9:20:36 PM
Amazon instances that were initially created outside a VPC, then migrated into a VPC, may receive 'Windows is not Activated' error messages.
To activate Windows, you should:
Download the latest EC2 Services
Run the following from an administrative command prompt:
cscript c:\windows\system32\slmgr.vbs -skms 169.254.169.251, 169.254.169.250, 79.125.16.172 or 79.125.16.108
cscript c:\windows\system32\slmgr.vbs -ato
Stop and restart the EC2Services
More details are at this AWS forum post.