Check Operating System Serial Key
ShowKeyPlus is a freeware which helps users retrieve the serial, license or product key of the installed Windows 10/8.1/8/7/Vista operating system.
Essentially all Microsoft programs require a product key as part of the installation process, including all of Microsoft's Windows operating systems.
All versions of Windows keep copies of the product keys used to install them in the Windows Registry but newer versions also encrypt them, meaning that finding them involves both knowing the location and how to decipher them.
Fortunately, programs called product key finders can do this all for you automatically, and usually in just a few seconds. Once you have your valid product key, you'll be able to reinstall Windows legally and be able to successfully activate it afterwards.
Since Microsoft changes how they encode and store product keys from each version of Windows to the next, there are preferred programs and methods depending on which version of Windows you have.
Find your version of Windows below, follow the linked how-to guide, and you'll have your valid Windows product key in no time. See What Version of Windows Do I Have? if you're not sure which to pick.
If something about using product keys in Windows is still confusing for you, or you're not sure if you even need to find your product key to install Windows again, see my Windows Product Keys FAQ for help.
Windows 8 & 8.1
If you've lost your Windows 8 product key but it's still installed or at least still on some sort of a working computer, it's pretty easy to decode with the right software.
See How to Find Your Windows 8 or 8.1 Product Key for an easy to follow tutorial.
While a lot of key finder programs advertise that they can find and decode your Windows 8 product key, I've found that many of them simply do not do it correctly, producing a completely inaccurate Windows 8 product key.
I've tested Belarc Advisor, the free program I suggest in my tutorial, and know that it will give you the correct Windows 8 key for your installation.
This procedure works equally well for any edition of either Windows 8 or Windows 8.1, plus Windows 8.1 Update.
Windows 7
Looking for your Windows 7 product key? Like with most other product keys, it's still around if Windows 7 is still installed, but is encrypted.
See How to Find Your Windows 7 Product Key for easy instructions.
Most key finder programs work great with Windows 7, but I prefer LicenseCrawler for several reasons.
The how-to guide I linked to above for Windows 7 keys works great with any edition of Windows 7, including Ultimate, Professional, Home Premium, and more.
Find Operating System Key
Both 32-bit and 64-bit versions are also equally supported. This goes for most versions of Windows and the key finders that support them, Windows 7 or otherwise.
Windows Vista
As unpopular as Windows Vista was, most product key finder tools support the operating system.
Like other recent versions of Windows, you'll have to use one of these programs to find Vista's product key because it's encrypted:
LicneseCrawler works great for Vista as well as Windows 7 (above), but just about all the programs in my key finder tools list will work just fine.
You might find a key finder or two that skipped Vista support, but it's not common.
Windows XP
Windows XP was the first consumer-focused operating system to encrypt product keys and, in general, to take the product key process very seriously.
So, unlike with older versions of Windows (a few sections below), Windows XP forces you to use those special product key finding software tools if you want to dig up your lost XP key.
See my How to Find Your Windows XP Product Key for a complete tutorial on this process.
There are a few programs I've grown to prefer when looking for product keys on my customer's computers, most of these tools fully support any edition of Windows XP. This really isn't that surprising knowing that XP was the version of Windows that prompted these tools into development.
Windows Server 2012, 2008, 2003, etc.
Considering how costly they are, it's no surprise that Microsoft has always required a product key for their Windows Server line of products, like Windows Server 2012, Windows Server 2008, and Windows Server 2003.
Not all product key finder programs support Microsoft's server-class operating systems, so there are fewer of these programs you can rely on.
See How to Find Windows Server Product Keys for detailed help.
This tutorial works for any of Microsoft's business-class operating systems, including the Windows Server versions already mentioned, plus Windows 2000 and Windows NT 4.
Windows 98, 95, & ME
Unlike in all newer versions of Windows, the product keys used to install Windows 98, Windows 95, and Windows ME are not encrypted in the Windows Registry.
This makes finding them really, really easy... so long as you know exactly where to look.
Check Operating System 32 Or 64-bit
See How to Find Lost Product Keys for Windows 98, 95, & ME for detailed help.
You'll need to open and use Registry Editor to do this, but don't worry, you won't be making any changes to the registry or doing anything dangerous.
Check Operating System Serial Key Free
While you may have a good reason for installing or reinstalling a very old version of Windows like Windows 98, etc., please know that these operating systems have serious security flaws and should not be connected to the internet.