So, it's probably one of motherboard, cpu, or PSU. At a stretch, it could be the GPU.
I have another spare GPU I could swap in. I have a near-identical CPU that I could swap in (it's in use, but I can temporarily borrow it).
PSU and mobo are trickier.
So, I'll have to try the easy ones first. Swap the GPU and see if windows still hard crashes like that, then the cpu, then start working on the others.