There are many other reasons why despite having an optical drive, you might want a bootable USB device for installing Windows. One is that installing Windows from a flash drive is a lot faster than that from a DVD. Creating a flash drive for installing Windows is not as simple as copying the setup files to it as it will not be bootable. To make it bootable, you need to use special software.
There are many third party software to create bootable USB devices but given the fact that most people aren't comfortable with third party software, Microsoft has released an application called Windows 7 USB/DVD download tool. Although the program's name makes it feel that it only supports Windows 7, that isn't really the case. It works well for preparing a bootable USB device for installing Windows 8 and Windows 8.1.
Before you begin, you must make sure that your flash drive has a storage capacity of at least 4GB. Also, you need to make sure that you have an ISO file of the Windows setup DVD with you. If you do not have the ISO file, then you need to create an ISO file from your setup DVD using a free program like ISOCreator.
Steps to Install Windows from USB flash drive
1. Install Windows 7 USB/DVD download tool. To install this program, you need .NET Framework version 2.0 or higher. The program runs perfectly on Windows 8, Windows 8.1, Windows 7, Windows Vista and Windows XP. Also, you need to be in an administrator account.
2. Open the program. Enter the location of your Windows ISO file in the source file box. Alternatively, you can browse to it and click Next.
3. On the choose media type screen, select USB device to create a bootable Windows installation flash drive.
4. On the insert USB device screen, select your USB flash drive or the external hard drive to which you want to copy the setup files and click Begin copying. If your USB device does not have enough space, you will be shown a dialog box saying so. Click Erase USB device. This will format your USB device and delete all data on it.
Tip: If you accidentally delete the data in your USB device in the previous step, you could still recover it with free tools.
5. If everything goes right, you will see a Bootable USB device created successfully message on the next screen.
6. Now, you need to boot from the USB device you just created to start the Windows installation process. If your computer doesn't boot from it, you might need to change the boot order. Also, you must note that older computers won't boot from a USB drive that is formatted as NTFS which is what the Windows 7 USB/DVD download tool formats it into while you select Erase USB device option. To overcome this, you should make sure that your device is formatted as FAT32 and has enough disk space.
If it complains that it is not a valid ISO file, you need to make sure that your Windows ISO is original and has not been modified in any way. If you are still unable to install Windows from a USB drive, just add a comment.
