I have a Dell Inspiron 15 7000 Series 7559 laptop that came pre-loaded with Windows 10. The Anniversary update for WIn 10 refuses to install with error code 0x80070003 and I have tried multiple things from online research all to no avail. Tired of the thing constantly downloading the update for it to fail... Sick of Win 10 frankly as well as it truly is horrid. My question: is there anything preventing me from reformatting my C Drive and installing Win 7 Ultimate x64 on this laptop model? I assume I could pick up the needed Win 7 drivers from the Dell site. Please advise.
It can be done, but because Microsoft removed the ability to boot Windows 7 USB media on Skylake platform system (which your system is), it is going to require the preparation of a new set of boot media -- you cannot simply make a Windows 7 or 7-SP1 boot medium and do the install. It isn't a trivial exercise. And because Windows 7 lacks support for the advanced power management of the Skylake platform, the system will run hotter and with reduced battery runtimes.
You might be better off doing a clean install of Windows 10 including the anniversary update (the ISOs are available from Microsoft) than in downgrading to Windows 7. Also, unless you have 10 Pro, you don't have downgrade rights so you'll need to have an unused license key for Windows 7 (they're no longer legally available for sale).
For the process, see
To do a clean install of Windows 10 Anniversary Update from ISO wouldn't I need a new key also since I would be blowing away the current installation? And unlike with Windows 7 where I have a number of extra keys around, I do not own any Win 10 keys outside of the one that came with the laptop. I refused to upgrade any of my other current Windows 7 machines to Windows 10 it is so bad with the downloading constantly and privacy issues.
You have a Windows 10 OEM System Locked Preinstallation key embedded in your UEFI BIOS. It will automatically be input by Windows 10 RS1 Installation Media. For more details see here:
Clean installing Windows 7 is going to require additional driver support. The procedure as mentioned is relatively complicated and you can only use OEM downgrade rights if you bought the system with Windows 10 Pro. See here for details: