How to diagnose startup crashes on a new rendering PC before Windows login?

[AndrewKL]

I've built a new PC for 3D rendering work, and after a successful initial setup and a few days of stable use, it's now experiencing consistent crashes on startup. It powers on, I see the motherboard logo, and then it either freezes completely or reboots itself before reaching the Windows login screen. I've tried booting into safe mode, but it crashes before I can access it. For anyone who has tackled similar instability, where should I begin my troubleshooting? I'm wondering if this points to a RAM compatibility issue, a problem with the power supply not delivering stable voltage under load, or a corrupted Windows installation, and what's the most systematic way to isolate the faulty component when I can't even get into the operating system to run diagnostics?