To reformat and reinstall is the computer equivalent of erasing the chalkboard and starting over with a completely blank slate.
To a computer, that means erasing the hard disk and starting over with nothing.
That phrase “erasing the hard disk” is something exceptionally important – and worth understanding.
This article applies mostly to Windows 10, though Windows 8 is similar. I have an older article covering Windows XP and Vista, which can also be used to extrapolate what needs to be done for Windows 7. In all cases, the concepts are very similar.
Reformat and reinstall
Reformatting a machine and reinstalling everything to start over is actually a five-step process for most people.
I can’t stress the importance of step #1: backing up. We’re about to erase everything on your hard disk. Unless you know for certain there’s nothing there that you want, an image backup is the right way to be prepared when you remember some weeks from now that you forgot to save a specific file.
Formatting isn’t always formatting
The term “formatting” actually goes back to days when hard disks had to be physically prepared before any data could be written on them. Formatting set up the underlying magnetic information so the disk could be used. The term also referred to the next step: setting up the initial data that would organize the files and folders to be added later.
Coincidentally, just that last step – setting up the initial data on the disk – has the side effect of making anything that might have previously been on the disk inaccessible. As a result, “reformatting” has come to mean exactly that: erasing the disk by setting up those initial data structures.
For the purposes of reinstalling Windows from scratch, all we really care about is that the disk be erased before we start. There are two approaches.
- Use a utility like DBAN to erase the disk before running Windows Setup. This is only really useful if you want to completely and securely erase all traces of data on the disk before.
- Let Windows Setup erase the disk for you. This is plenty for most situations.
The “trick”, if you want to call it that, is that it’s not always obvious what to do in Windows Setup.
So I’ll show you.
Step one is to boot from your installation media.
Exactly how you do that1 will vary dramatically based on your specific computer. If your computer has the UEFI BIOS, which includes things like “secure boot”, this article applies. If your computer is older and has a traditional BIOS, check out this article.
Once you boot, you’ll be asked to choose your language and keyboard; then click Next.
Click on Install Now to begin the actual setup process.
If you have a product key, you can enter it at this time, or click on I don’t have a product key to do so later. Click on Next.
This particular set-up disk applies to Windows 10 Home and Pro both. Make your selection here, and click Next.
Naturally, you’ll need to accept the product license. Click the “I accept the license terms” checkbox, and click on Next.
Now choose the type of installation to perform. Click on Custom.
Windows Setup asks where you want to put your new installation. This is where the reformatting magic happens. In order to “reformat” the disk, you will delete the existing partition(s) on the disk and let Windows Setup create new ones.
The dialog box lists all existing partitions on the hard disk. In this example, you can see that there are two: System Reserved, and a primary partition.
There are two approaches you can take.
- Delete only the partition you know contained the previous installation. Typically, this is the largest partition on the hard drive, and is marked “Primary”. If you’re not certain, this is likely your best option, so as not to delete important UEFI or manufacturer’s recovery partitions.
- Delete all the partitions on the hard drive. This will allow Windows Setup to make maximum use of the hard drive, and is typically safe in older, pre-UEFI machines.
To delete a partition, simply click on the partition in the list and click on Delete. The partition should either become “unallocated space”, or be merged with any adjacent unallocated space.
Once you’re done deleting partitions, click on the unallocated space item into which you want Windows installed. (Typically the largest, if there’s more than one.)
Ironically, you do not need to format the space into a partition, even though the option is there. Windows Setup will take care of the rest. Click Next.
At this point, Windows Setup proceeds to install Windows normally.
- How do I reformat and reinstall Windows? (XP & Vista) – Sometimes the best approach to resolving an issue, be it a virus or simply software rot, is reformat and reinstall. We’ll review the steps.
- Where can I download Windows? – You can download Windows versions 7, 8.1, and 10 from legitimate sources. If you have a legal retail product key, this could be your replacement media.
- How do I reformat my machine if I don’t have a Windows CD? – Reformatting a machine is a major step that erases everything. Unless you really know what you’re doing, you’re going to want that Windows Setup CD.