Note: If it’s been fewer than 10 days since you upgraded to Windows 10, your previous version of Windows will be listed as a system file you can delete.If you need to free up drive space, you can delete it, but keep in mind that you'll be deleting your Windows.old folder, which contains files that give you the option to go back to your previous version of Windows.
Then, go to the ‘advanced BIOS’ configuration options and navigate to the boot options. Set the Windows 10 installer (could be a bootable flash drive or CD) as the first boot, and the Hard Disk Drive as the second boot. Press F10 to save changes and reboot. As the computer restarts, make sure to press any keys to load the installer.

Windows does not allow users to format C drive when system is running, for formatting C drive removes operating system, installed applications and other data on the partition. In this case, we need to complete the task outside of Windows. In this guide, you can find several ways to format C drive (system partition) in Windows 10/8/7. The solution is to format C from outside of Windows, meaning you need a way to do it from somewhere other than your Windows installation. The easiest method is to boot from an operating system (with formatting abilities) via a CD/DVD/BD drive, flash drive, or floppy drive.

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Here's how to create a bootable Windows installation USB drive starting with a Windows .iso file or a Windows Setup DVD.

Tip

If you're looking for an easy way to create a bootable USB Windows installation drive, use the media creation tool.

What you need

Windows 10 install .iso or DVD
USB flash drive with at least 5GB free space. This drive will be formatted, so make sure it doesn't have any important files on it.
Technician PC - Windows PC that you'll use to format the USB flash drive
Destination PC - A PC that you'll install Windows on

Step 1 - Format the drive and set the primary partition as active

Connect the USB flash drive to your technician PC.
Open Disk Management: Right-click on Start and choose Disk Management.
Format the partition: Right-click the USB drive partition and choose Format. Select the FAT32 file system to be able to boot either BIOS-based or UEFI-based PCs.
Set the partition as active: Right-click the USB drive partition and click Mark Partition as Active.
Note
If Mark Partition as Active isn't available, you can instead use diskpart to select the partition and mark it active.

Step 2 - Copy Windows Setup to the USB flash drive

Use File Explorer to copy and paste the entire contents of the Windows product DVD or ISO to the USB flash drive.
Optional: add an unattend file to automate the installation process. For more information, see Automate Windows Setup.

Step 3 - Install Windows to the new PC

Connect the USB flash drive to a new PC.
Turn on the PC and press the key that opens the boot-device selection menu for the computer, such as the Esc/F10/F12 keys. Select the option that boots the PC from the USB flash drive.
Windows Setup starts. Follow the instructions to install Windows.
Remove the USB flash drive.

If your Windows image is larger than 4GB

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Windows USB install drives are formatted as FAT32, which has a 4GB filesize limit. If your image is larger than the filesize limit:

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Copy everything except the Windows image file (sourcesinstall.wim) to the USB drive (either drag and drop, or use this command, where D: is the mounted ISO and E: is the USB flash drive.)
Split the Windows image file into smaller files, and put the smaller files onto the USB drive:
Note, Windows Setup automatically installs from this file, so long as you name it install.swm.