When you reinstall Windows, you can use the same Windows product key that came with your laptop or desktop PC. You'll often find it printed on a Certificate of Authenticity (COA) sticker on the PC itself. If you can't find it, run a program like NirSoft's free ProduKey to find the product key associated with your installed copy of Windows, and write it down ahead of time. Many new Windows PCs come with their product key embedded in their UEFI firmware and should automatically enter the key when you install Windows 8, so you won't always have to do this by hand.
There are downsides to this route. You'll have to spend some time reinstalling Windows whenever you get a new PC rather than using it immediately. Any files and programs currently installed on your PC will be erased, so this method should be used immediately after you get a new PC, not for cleaning up leftover bloatware weeks later.
You'll also have to install any useful hardware drivers or utilities afterward. Windows will automatically fetch many hardware drivers and install them, but it won't necessarily install everything. You'll want to visit your hardware manufacturer's website, locate the page for your specific model of computer, and download the appropriate drivers for it. (Here's a guide to finding tricky drivers if you run into trouble.)
Burn it down to build it up
No matter which method you use to combat bloatware, the fun part begins when you're done scraping out all the crap: Loading your PC back up with genuinely useful software. We can recommend a number of truly useful software and utilities that everyone should have.
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