When a computer starts, it loads the BIOS sequence, then it will boot the operating system from a location (hard drive, DVD/CD-ROM, USB ..).
The boot location is defined in BIOS (also called CMOS) menu.
The BIOS sets the boot order, this means it will look on those locations and if will find a valid file, it will start it from there.
For example, if you have the Hard Drive first (in the boot order), the computer will start the Windows operating system. If you have a bootable DVD/CD or USB and you want to reinstall Windows, you can’t because the Hard Drive is first and the computer will load the installed operating system and will not check for the next devices mentioned in the boot order. So, to install a new Windows from DVD/CD or USB, you must put these devices first, in BIOS->Boot order.
Tip: in order to start the PC faster, let the Hard Drive first at the boot order, after you finish your new Windows installation from a CD, USB etc..
1 Go to BIOS
See here how to enter in BIOS menu. Usually, press Del or F2 when the computer starts.
3 Change boot order
Then press + or – to move it up or down.