I had the same sort of situation with a HP desktop my in laws gave me. They never used it and it's an i7 beast. It came with W7 home but I wanted to install an SSD drive as the machine was a mess from to many grandchildren messing about. 
 
HP like Dell offers you a W7 install CD but wanted $50? I have a crack of W7 and so I desided to go that route as after all, the machine had a paid for licence but was locked to the old HD partition. No guilt. 
But as said above the W7 initial install was easy,,, but- it took days and and days to update. It became stupid. And I had driver issues and the USB 3 ports and the SD card reader etc did not have any. 
 
So I gave up and desided I would just buy W10 and be done with it. This was the Fall after the free deadline. But right on the MS web site they tell you how to get W10 for free and so I did that. So any how 
You can try the W7 install and because you do have the proper install software you might not have the trouble I had with the drivers. 
But I was going to tell you if you have to buy an new OS I agree that W 8.1 is what I would choose. 
You can still get W10 for free. looks like it's finally going to expire year end. 
 
https://www.cnet.com/how-to/microsoft-windows-10-free-upgrade-offer-assistive-features/
 
 
 
 

Billy86, you don't need to buy another Win 10 to do a clean install.  You already "own" Win 10 and if you sign in to Win 10 with a Microsoft Account (e.g. I have a Hotmail account and use that) then you have "digital entitlement".  All you need to do is download Win 10 and create a bootable media -DVD or a USB Flash Drive.
 
You don't need any Dell drivers either, Windows is providing the latest driver versions.
 
Here is the instructions from the www.tenforums.com  Tutorials (use Option 1 Media Creation Tool).
https://www.tenforums.com/tutorials/9230-download-windows-10-iso-file.html
 

You don't need any Dell drivers either, Windows is providing the latest driver versions.  
 
Are you sure of that Jack? I found that this was a huge issue for 2 of my computers where there ended up being drivers missing or not up to snuff. On one machine the video driver W10 installed was so basic I couldn't even change screen resolution. Luckily I still had the CD.  My wifes laptop she lost her Card reader, camera and mike and nothing can fix that so she can't use it for skype anymore.  
It's from my observation one thing W10 does a pour job of. And many have reported W10 updates overwritting proper drivers with crappy generiic drivers. 
 
 
 

Okay, all. Like I said: Hmmm.... nothing's easy.

If you took the free upgrade From Windows 7 to Windows 10 you have probably burned that Win 7 license, in exchange for the upgrade.  There was an initial 30 day rollback period, but that has long since expired.  You probably cannot re-activate the Win 7 license that came with that PC.
 
No need to buy a fresh copy of Win 10 to test it out!  Windows 10 has relaxed the grace period that you have after installing a fresh copy.  You now have a lifetime grace period!  You will only see a nag screen and have limited customization options, else the Windows is fully functional!  https://www.windowscentral.com/you-do-not-need-activate-windows-10
 
Get Win 10 here >> https://www.microsoft.com/en-us/software-download/windows10

Billy86 I can buy a copy of Win 10 to do the fresh install onto the SSD, or I can rollback to factory fresh, Win 7, as it was in 2012 when Dell shipped it. I'd install that on the SSD, along with Sonar and related.
 
Not sure what route to take, if I do a fresh install. 

If you're prepared to purchase Windows and create a new install, why not buy Windows 8.1 instead? It's easier to deal with and configure than Win 10, newer than Win 7, and runs very nicely as a DAW OS - I'm using it for my workstation. There are reputable dealers on eBay and elsewhere who will sell you a new Win 8/8.1 product key that is affordable. And then no more Win 10 headaches.  
 

I've installed/reinstalled Win 10 on several computers, including my DAW desktop and a Lenovo and a Dell laptop.  Everytime, Windows (as long as you are connected to the internet) installed all the needed device drivers.
 
There are probably "exceptions" but as long as the hardware is Win 10 compatible most get everything needed installed.
 
With older OS', such as Win 7 or even Win 8 that was not the case. As much as many hate Win 10, its been "painless" for me.
 
 

I tried putting Win10 on a Mac a few months ago. The idea being a dual-boot system so I could run a few Windows games and a few small Windows only applications.

So I downloaded the relevant Apple Boot Camp drivers for Win 10, installed it and ran Windows update. After it had finished and rebooted I found it had replaced some of the Apple drivers, including the video one, with generic Windows drivers.

So off came Win10 and on went Win7. End of problems.

Some drivers are ONLY Windows/Microsoft drivers.  e.g. drivers for optical drives (CD/DVD) are only the default built in Windows drivers.  Hard drives the same way.  Some USB drivers are only the Windows/Microsoft drivers.  If there is no hardware sound driver available and its still compatible with Win 10 then the generic, basic function, High Definition Audio driver will be installed.

Use Win10 Pro.
There are registry tweaks that completely shut down all automatic updates.
You can even turn off notifications...
The Pro version also has the Group Policy Editor... which allows turning off Cortana, OneDrive, etc... without going into the registry.
 
Win10 is a fine DAW platform.
Just need to take the above steps... and you're good-to-go.