1. Please, shutdown the computer using the power button
2. Power it on.
2.1 As soon as you press the power-on button, keep hitting the Esc button (like tap-tap-tap).
This should open a HP Startup Menu which may look similar to this one
3. Choose F9 for Boot Device Options
4. Select your HP media (DVD or USB) as bootable device to boot from.
5. Follow the on-screen instructions
Please, let me know if this works.
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