Disk usage 100% since upgrade to Windows 10

At work I am using a Dell Optiplex 3020. This machine was fine until I upgraded to Windows 10; now the PC is pretty much unusable. It freezes up every 15 second or so and I can’t seem to work it out. The reason it is freezing is because it is saying that the disk usage is 100%.

I have tried a clean install, I’ve tried running all maintenance and diagnostics from the dell site, updated all drivers that dell recommended, disabled super fetch, windows search, live tiles, windows defender (downloaded avast), stopped it defragging and pretty much every thing else I’ve found online but I cannot fix this.

I’m about ready to give up and reinstall Windows 7, but now I can’t find my product key, as Windows 10 uses a different format and I don’t think I can get my Windows 7 product key through Windows 10 (yes I should have noted it down first)

If anyone can help in how to either fix this issue or find my product key I would be grateful.