With Windows 8/8.1 The Product Key on preinstalled Operating Systems (HP, Acer, etc) is embedded in the BIOS in the Motherboard, so that the lost and/or worn C.O.A. Sticker problem is no more.
This information may interest you if you do wish to keep a record of it:
“Windows 10 Embedded Product Key Tool”
“We’re proud to introduce the immediate release of our newest utility for Windows users: the NeoSmart Technologies Embedded Product Key Tool. Designed for users of Windows 7, Windows 8, and Windows 10, our embedded product key tool will retrieve and display the Windows setup product key embedded in the BIOS or EFI, allowing you to store it for safe-keeping or use it to reinstall Windows with an official Windows setup image.”
You can also use:
“Belarc Advisor – Free Personal PC Audit (Version 8.5c)”