Solved: Boot From CD Dell Inspiron 15 3000 series
One of my customers had a Dell Inspiron 15 3000 series laptop with a service tag cv6nn52.
When I started to install Windows 7 on it instead of the Windows 8 it came with I didn't find the option to boot from CD on the start menu nor on the BIOS BOOT menu.
Luckily there was no error - it's just a simple security measure that has booting from CD disabled by default from BIOS.
To enable booting from CD on Dell Inspiron 15 3000 series go like this:
2. When bootings start tap on F2 to enter BIOS
3. Navigate to Boot menu on the top of the page
4. Go to Secure Boot and select Disabled
5. Navigate to Save & Exit at the top of the menu (the last menu item) and reboot.
6. When the starts to boot, tap on F12 to enter boot menu.
Voila! Now you can see CD/DVD option in the boot menu and can proceed installing your new OS from the CD.