Windows Proof of of Licence sticker?

Windows 8 doesn't need them depending on how it's built. We don't have them at work as each PC connects to microsoft to validate the license.

 
The important item is the Product Key (PK) which was typically printed on the Certificate of Authenticity (CoA).  Many Windows 8 computers have the PK embedded in the motherboard firmware, making the CoA unnecessary.

It would still be prudent to record the PK, which is recorded in the system registry (on your hard disk).  You can use NirSoft's ProduKey to retrieve and decode the PK.

http://www.nirsoft.net/utils/product_cd_key_viewer.html