You can find the product key currently in-use on your Windows PC with a tool like NirSoft’s ProduKey. If you don’t have a disc, you can download Windows 10 installation media from Microsoft or download a Windows 7 ISO file from Digital River. ( UPDATE 2/25/15 Digital River no longer works you can now download Windows 7 ISOs from Microsoft’s Software Recovery website.) You can still get Windows 8.1 installation media from Microsoft, too. If you’ve built your own PC and installed Windows on it, you should have a Windows disc lying around. You’ll have to remove unwanted junk software after you reinstall. All the original drivers will be installed, which is good, but all that nasty bloatware will also come back-that’s bad. You’ll end up with the manufacturer’s like-new Windows system on your drive. If your computer comes with a recovery disc, you can also insert it in your computer’s optical drive and boot from it to begin reinstalling Windows. It should also be printed in your computer’s manual. This key may be displayed on your screen. On many PCs, you’ll have to press a key during the boot process to access the recovery tool. If your computer has a recovery partition, run your manufacturer’s recovery tool to reinstall Windows. Most manufacturers don’t include Windows installation discs with their computers. In Windows 7 and previous versions of Windows, it’s up to the PC manufacturer to provide a recovery partition or recovery discs. Use your manufacturer’s recovery partition or discs (Windows 7 or earlier)
With the Anniversary Update, Microsoft may allow Windows 10 users to reinstall Windows 10 and remove the manufacturer-installed junk much more easily. Look for a “Learn how to start fresh with a clean installation of Windows” option at the bottom of the Recovery pane after upgrading to the Anniversary Update. It should be just as good as installing Windows 10 from scratch. In Windows 10’s Anniversary Update, Microsoft is experimenting with a new “Give your PC a fresh start” tool that will allow you to reinstall Windows from here, erasing even all that manufacturer-provided junk. On Windows 8, both of these options are available in the modern PC settings app under Update and recovery > Recovery.
You can also access these options by booting from a Windows recovery drive. If your computer isn’t booting properly, it will boot to the advanced startup options menu, where you can select Troubleshoot to reset your PC.
You can then tell Windows to Keep my files or Remove everything. Click or tap Get Started under Reset this PC. On Windows 10, this option is available in the Settings app under Update & security > Recovery. You can choose whether you want to keep your personal files or not. Resetting this PC will delete all your installed programs.