(Redirected) Inspiron 660s Factory Settings

After a recent Windows 10 upgrade, my PC slowed down dramaticall, so I rolled back to Windows 8, then upgraded to Windows 8.1. My teenage son tried to speed up the response time and made matters worse. Now my PC won't boot-up and I can't regain my pre-installed OS. I kept getting the DELL symbol then a black screen. I have tried all the tactics I found online...to no avail. Some being...

Start + Ctrl + F11 NOTHING

F8 on bootup NOTHING

Change DVD from UEFI to LEGACY NOTHING

Ctrl + F11 NOTHING

I ran diagnostics with all green ticks...that's a good sign. I can't get it to boot in any DVD or USB. I have tried every DVD I own (I own all the Windows). How do I restore my PC back to factory settings? Does anyone know the magic key? Thanks.Inspiron 660