Help! My girlfriend’s PC posts but won’t boot up

I built my girlfriend a PC a few months back, and now she's telling me it won't boot. The system posts just fine, but it blue screens as soon as it attempts to start up. I initially suspected it was an OS issue, so I tried using my installation USB, but it won't boot from that either. I've already swapped the RAM, removed the GPU, and unplugged both hard drives to check for corruption. I've also tried every BIOS setting I can think of, reset the CMOS, and reseated the CPU. Unfortunately, I don't have a working CPU to test it with – just two non-working ones. I'm at a loss here. When I switch between CSM and UEFI modes, I get different graphical glitches: in UEFI, the MSI logo appears cut in half, and in CSM mode, it looks like the logo is shredded across the screen. Any suggestions?