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have. For Windows 7, right-click Computer, select System and note down the edition, such as Windows 7 Pro. Note if it's 32-bit or 64-bit by looking at the System type section. For Windows 8 or 8.1, press Windows-X, click System and note down the information.

Use Belarc Advisor (free from www.belarc.com/free_download.html) to find the Windows licence key.

To download Windows 8, you need to download the installer file from the Windows 8 page at tinyurl.com/productkey. Make sure you get the Windows 8 version, not the Windows 8.1 version. Run the software, enter your CD key and click Next. When Windows 8 has downloaded, select ‘Install by creating media’ and click Next. You can now choose to create a bootable USB flash drive or an ISO file, which will download the file to your hard disk so you can install it later. The latter gives you more flexibility. If you choose to save the ISO file, use Windows Explorer to pick a save location. It can take a while to complete your selection, so just wait until it's complete

Windows 8.1 is a little easier, as you don’t need your product key at this point. Go to the Windows 8.1 media page (tinyurl.com/win8install), download the installer file and run it on your computer. Use the drop-down menus to choose the language and version of Windows 8.1 you have and click Next. You can now choose to create a bootable USB flash drive (you’ll need a 3GB or larger drive plugged into your computer) or download the ISO, which gives you more flexibility. Once you've selected your option click Next; if you choose to save an ISO file, use Windows Explorer to pick a location. Again, this can take a while.

Microsoft doesn't provide an official way to download Windows 7 installation files, but its partner Digital River does. All you have to do is download the version you've got, using the download links below. Sometimes these links go down and theoretically they could be removed at any time, but we've always found them to be a stable and reliable way of getting the ISO files.

Windows 7 Home Premium X86:

Windows 7 Home Premium x64:

Windows 7 Professional :86:

Windows 7 Pro x64:

Windows 7 Ultimate x86:

Windows 7 Ultimate x64:

if you used the Windows 8 or Windows 8.1 tools to create a bootable USB drive, you're ready to go.

If you have Windows 7 or you opted to download the ISO file, you now need to do something with it.

The easiest thing to do is to use the Microsoft Windows USB/DVD Download Tool (wudt.codeplex.com) and install it on your computer. Run it, point it at the ISO file you downloaded and then choose if you want to create a DVD or USB bootable device. Follow the instructions through to create your install media.