Finally, figured how to install Windows 7 32/64, all editions on numerous NUC's!
Previously asked NUC6i5SYH - Windows 7 64-bit - No Peripherals , and half-satisfied with the information provided, and have provided a 100% working fix.
Tested on the following NUC models: NUC6i5SYH, NUC6i3SYH, NUC5i7RYH, NUC5i3RYK, and NUC5PPYH.
The NUC's were either mine or a friend's and ran Windows 7 flawlessly for 14 days. If on the fence on NUC, these are definitely a replacement for desktop from lite computer tasks all the way to running VS 2015, eclipse, Sony Vegas, and photoshop CS6 at the SAME time. Gaming, I have no idea. Tetris anyone?
***YOU NEED A STANDARD USB 2 KEYBOARD FOR THIS GUIDE!***
Windows 7 sp1 ISO
RT 7 Lite (if making a custom/slim install)
Microsoft updates (kernelmode driver framework 1.1, usermode driver framework 1.1, optional NVME driver update if using M.2 drives), and generic MS Windows 10 drivers.
NVME driver update: https://support.microsoft.com/en-us/kb/2990941
DO NOT USE the Intel drivers (sorry Intel), as you'll be limited to the NUC's that those drivers support. With the Windows 10 generic drivers, you'll be able to install on ANY NUC very easily.
Windows 10 NVME driver did not "port" well so I'm forced to using the one provided by their update. You can extract the drivers from the NVME update.msu but I'm not explaining that in this guide.
DISM GUI 4.0, and 7zip.
My RT 7 Lite settings here: RT7Lite (includes the installer, registry file changes for me (welcome to use, should work fine with any install)
NOTE: the RT7Lite file on google drive includes registry file in a zip folder, DO NOT USE THAT, its outdated. That folder is only for adding custom images/sounds to my ISO's. While RT 7 Lite does come with the necessary WAIK files, I HIGHLY suggest downloading Download The Windows® Automated Installation Kit (AIK) for Windows® 7 from Official Microsoft Download Center anyways, as it provides a more stable ISO editing process.
To obtain the Windows 10 drivers, I used Slimware driver utility to extract the updated drivers bundled with Windows 10: HID mouse/keyboard (2015), USB 3 drivers (xhci 2015), MS mouse/keyboard detection drivers(2015). Or download them from my google drive: MS Win7 Updates & Win10 Drivers (bundled with updates mentioned above, x86/x64).
NOTE: Force unsigned and Recurse both need to be checked before applying drivers.
Step 1: Extract ISO Files
Extract ISO files with 7z to a folder on desktop, for this guide we'll name it Win7.. While we're at it, create a new folder named mount.
Once the files are extracted, go to "Step DISM" if you're not creating a custom ISO.
Step 2: RT 7 Lite
Follow my guide, Reduce Windows 7 OS Size - all versions - Windows 7,(slightly outdated with RT 7 settings, see above to download, you'll manually go through and check off things, services I've labeled in english..)
My x86 ISO installs and uses just over 1.55GB disk space, no errors/bugs of any kind. My x64 ISO installs and takes only 3.3GB space.
(If Intel needs proof that it's my guide I'm sharing above, happy to provide it.)
Step 3: Pre-ISO
After the changes are saved to your ISO folder, before you actually finish creating the ISO, start DISM GUI.
Step 4: DISM
Locate the boot.wim, and set the mount folder to where you have it saved on desktop from earlier. Click "Display WIM Info" and make note of the Index number. If you made a custom ISO with RT 7 Lite, the default for all editions edited is index: 1.
Go to the "Driver Management" tab, find the drivers folder, and click "Apply." Now go back to first tab "Mount control" and click "Dismount WIM." It'll ask to apply changes, click "Yes."
Don't close out of DISM yet. In the WIM file location, replace "boot.wim" with install.wim" and perform Step 4 again for the install.wim file.
Step 5: 7zip
Start 7zip, and navigate into boot.wim and install.wim, for both files have a new folder under the "Windows" folder called "[DELETED]." Ironically named, we'll be deleting these "deleted" folder in both boot.wim and install.wim, kindly left behind by DISM GUI.
Step 6: Create ISO
If using RT 7 lite, create the ISO, and use 7zip to extract it to a USB 2.0 flash drive. The Microsoft Windows 7 usb tool is another option, but it literally does the EXACT same thing...extracts ISO files to USB. For the sake of downloading more stuff, I'm sticking to 7zip.
Go to your Intel NUC support page, and find how to access the BIOS, disable UEFI, enable Legacy. (Now on the 5th gen NUCs I ran into a very strange issue. You may find that disabling the BACK USB ports in BIOS will allow you to keep UEFI turned on, install Windows, and then enable the back USB ports).
Have the USB drive containing your Windows 7 install plugged in on the FRONT. Your standard USB wired keyboard will be plugged into the FRONT USB port on your NUC. Upon reboot, press the F# key to select your boot device, and away we go!
At this time, you may or may not have mouse support, which is perfectly fine. Use the keyboard "tab/arrows" to navigate until the first desktop setup completes.
Once installed, if you used RT 7 lite and made any registry changes or user-interface tweaks, YOU MUST REBOOT.
After rebooting, right click recycle bin, properties, and select "Don't remove files to recycle...." as this saves space on a SSD, and can help a HDD not defragment as frequently if you constantly install/uninstall software.
Enjoy your Windows 7 on NUC!
If you're a Linux user, the following distro's I've already tested:
Mint, Linux Lite, CentOS, Kali, Porteus, Backbox, ReactOS, Tails 2.3, Lubuntu, and KaOS.
All of the distros tested are the most recent stable release, and tested on each of the NUC's listed at the beginning.
Thanks for taking the time to read all this, and I hope this guide has helped. Providing links to my google drive folder containing VERY slim pre-built ISO x86/64 at a later time.