Product keys are one of the ways software developers protect their products against piracy. Unfortunately, they're easy to lose, which can be a real problem if you need to reinstall Windows or any other software from scratch

If you've bought a desktop PC or laptop with Windows 10 already installed, you'll probably find its product key on a Certificate of Authenticity sticker on the the case or in the included paperwork, but not if you've built your own PC and installed the operating system yourself.

Many other applications also require product keys if you reinstall them – including Microsoft Office – and if you've lost the email you received when you first installed them, you could be in real trouble later on.

Product key finders only work for software that's already installed, so download one and make a note of all your keys now, so you're prepared in advance.

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1. Belarc Advisor

A wealth of information about your hardware and SoftwareBelarc

(+) Automatic updates

(+) Lots of extra details

(-) No security score for Windows 10

Advisor is primarily a tool for gathering information about your Windows system, including hardware, security updates and product keys.

Whenever you run Belarc Advisor, it automatically checks its database for software definitions – essential for finding keys for new programs. After that, it scans your system and presents the findings in an HTML page in your default web browser. Scroll down to 'Software licenses' to find serial numbers and product keys for Windows, Office and many other applications.

Belarc Advisor offers a wealth of other information, including details of your antivirus software, connected hardware, and other devices on the same network. It also offers a security benchmark score, though sadly this is only available for Windows 7, Windows Vista and Windows XP Pro.

Belarc Advisor's scans are fast and offer so much more information than just serial numbers, it's our favorite free product key finder.

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2. Abelssoft MyKeyFinder

Displays your Microsoft product keys in a clear, searchable list

(+) Clean interface

(+) Searchable results

(-) Occasional ads in free version

There are two versions of Abelssoft MyKeyFinder: free and paid-for. Both will find Windows and Microsoft Office product keys but the Plus edition (£8.90 – about US$10, AU$15) will also scan external hard drives and find Wi-Fi passwords.

After MyKeyFinder has scanned scan your PC’s registry the results are presented in a clear list, ready to be copied to the clipboard with a single click. Unlike many product key finders, MyKeyFinder won’t throw up duplicate keys and the results can be searched and filtered.

You can also add programs and keys not detected by MyKeyFinder as standard, then export the full list as a PDF so they’re all in one convenient location.

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3. LicenseCrawler

Find the keys for Windows and a wealth of other Software

(+) Identifies dozens of applications

(+) Portable

(-) Slow scans

(-) Occasional ads

LicenseCrawler will find the license key for pretty much any application that has one, and it's free for home use. Its scan of the Windows registry takes a long time to complete – even if you select the 'high speed' option – but you can limit its scope using the blacklist and whitelist filters.

LicenseCrawler is distributed as a portable app, meaning you don't have to install it on the Windows system you're scanning – just download the zip file, extract its contents and run LicenseCrawler.exe. This is particularly handy if you have several PCs; just download it to a USB stick and you can just plug it in and run it on any of them.The only downside of LicenseCrawler is the occasional pop-up ad, but finding product keys isn't a task you'll be performing every day, so they're only a minor inconvenience.

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4. Windows Product Key Viewer

A quick way to find your Windows product key, but nothing else

(+) Super fast scans

(+) Portable app

(-) Only finds Windows keys

(-) Some misleading ads on site

Windows Product Key Viewer is super fast, taking barely a second to scan your system and present the results. That's because, as the name suggests, it only supplies the product key for your operating system.

That shouldn't necessarily put you off though. Although it doesn't find serial numbers for other applications, it does provide some extra information about your OS, including the registered user, install date and last boot time, and it's compatible with versions of the operating system from Windows 10 all the way back to Windows 95.

Take care when downloading the software – the site includes some ads that look like download buttons for the software, but are in fact unrelated. Only click the link 'Download Windows Product Key Viewer'.

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5. Free PC Audit

A super simple way to find your Windows 10 product key

(+) Very simple to use

(+) Portable

(-) Only finds Windows keys

Free PC Audit is another portable application, and doesn't even come as a zip archive – just download the exe file and run it to begin scanning.

Like Belarc Advisor, Free PC Advisor provides a wealth of information about your whole system – not just its software – though its interface isn't quite as intuitive. It's compatible with all versions of Windows, from NT all the way through to Windows 10, but doesn't find keys for Microsoft Office or any other software.

It also provides lists of all installed software and running system processes, though this information is readily available through Windows itself.