How to find a Windows product key
To install - or reinstall Windows 7 you need your Windows licence number or, to use its correct name, the product key. This might be printed on a sticker somewhere on your laptop or PC, or on some documentation provided with it.
If you don’t have this, or you’ve lost it, you should be able to extract the product key from your computer using a free utility called Magical Jelly Bean Keyfinder. This can also extract keys for other software. Obviously, you’ll need to have access to a computer running the very copy of Windows you want to reinstall, or install on a different computer. There are other utilities which can search your hard drive for keys, such as SterJo Key Finder, Belarc Advisor, and Recover Keys, the latter of which is a paid-for alternative which can extract keys for many more applications. It costs £19.95 but you can use the discount code PCADM20RK to get 20 percent off.
You won’t be able to get the product key from a PC or laptop if its hard drive has broken, or if Windows is corrupt and you can’t get it to boot, or you can’t log in.
How to install Windows from USB
Assuming you now have your product key, you will need to download Windows online.It's helpful to know which version of Windows your product key is for, such as Windows 7 Home Premium or Windows 7 Ultimate.
Step 1. To find out which version your computer is running go to the Control Panel, then System and Security, and finally System. Along with other information about your PC or laptop, this will tell you precisely which version of Windows 7 it is. Windows 7 keys should work with either 32- or 64-bit versions of Windows 7, but you can’t swap Home Premium to, say, Windows 7 Ultimate.
Step 2. To download Windows 7, head to Microsoft's website to download the correct version of Windows 7.
Step 3. Now you have the software you can burn it to a DVD or - easier for most people - extract it to a USB flash drive. You can’t simply copy the ISO file you downloaded to a USB drive or DVD. Instead, you need a tool to do this, such as Microsoft’s own USB/DVD tool
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Step 4. Shut down your computer and insert your USB flash drive (or DVD). Now power on the PC and it should boot from the disc or drive. If not, you’ll need to enter the BIOS and change the boot order to prioritise the DVD drive or USB drive. Here's how to enter the BIOS
Step 5. When you get your computer to boot from the disc or USB drive, you can install Windows 7. Be careful, though. If you’re planning to do a fresh install and format the hard drive as part of the process, make sure you’ve backed up all your documents, music, photos, videos and other files you don’t want to lose.
Step 6. Once your irreplaceable files are safely backed up, you can follow the on-screen prompts to install Windows. If you're asked if you want to upgrade or do a fresh (clean) installation, choose the latter if possible. This will wipe your hard drive and is the best choice for most people as it doesn't carry across all the detritus from your old Windows installation. This improves performance and uses less disk space. NOTE: If your hard disk is split up into several 'partitions' you will need to either delete them all using the options available during installation, or choose one of the partitions on which to install Windows.
You can delete or format (wipe clean) individual partitions, so if your data is all stored on the D: drive, say, and this is a separate partition on one physical hard drive, you can safely wipe the C: drive and install Windows on that partition without affecting your files. You will still have to reinstall all your apps and drivers, of course. As you can see, drive letters are not listed, so you'll have to go by the partition size. Just make sure you're certain before deleting or formattting anything.
For more, see How to format a hard drive