It depends what type of license you have as to whether it costs you money or not.
Retail and OEM licenses do not expire; but Volume Licenses have to be re-activated regularly and are not for private use.
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Activate Windows 7 or Windows 8.1 - Windows Help - Microsoft Support
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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.
When you reinstall it will pick up the Product Key and activate the system automatically.
If it does not or you have activation problems, follow these steps:
"How to Activate Windows 8 and 8.1 by Phone"
http://www.eightforums.com/tutorials/24636-activate-windows-8-phone.html
Follow these steps and do a Phone Activation and hold on for a real person to assist you.
OPTION TWO
1. Do step 2, 3, 4, 5, or 6 below for what you would like or able to do.
2. Press the Windows 
+ R to open the Rusluisn dialog, type
slui.exe 4, press Enter, and go to step 7 below.
3. Open an elevated command prompt, type
slui.exe 4, press Enter, and go to step 7 below.
4. Open the Power Users Tasks menu, click/tap
on System, and go to step 5A below.
5. Open the Control Panel (icons view), and click/tap
on the System icon.
6. Open PC settings, click/tap on Activate Windows on the left side.
Etc.
"Get help with activation errors"
http://windows.microsoft.com/en-us/windows-8/activation-errors
"How to activate a Microsoft product by using Microsoft Product Activation Center"
http://support.microsoft.com/kb/950929/en-us
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If phone activation is not available to you or does not let you contact Micosoft, you will have to use this method to contact MS support
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This information may interest you if you do wish to keep a record of it:
"Windows 10 Embedded Product Key Tool"
https://neosmart.net/blog/2014/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)"
http://www.belarc.com/free_download.html
Or:
"Product KeyFinder"
https://www.magicaljellybean.com/keyfinder/
Cheers.
