Subject says most of it. My HTPC will occasionally be found in this hard-locked/crashed state when nothing of note was being done. The past couple times, it has been while the screensaver was running over top of the Steam client (not Big Picture, just the normal client) and nothing else of note was running. I've not experienced this crash when watching TV via Windows Media Center, nor when playing Steam games like Dark Souls.
Checking Event Viewer has been fruitless, as no BugChecks or other errors are found, aside from the Kernel Power errors saying the previous shutdown was unexpected. Presumably, whatever happens is happening suddenly enough that the OS doesn't have time to react before it's locked.
I was also getting random reboots with the same level of mystery and non-existent reproduction steps. I turned off the auto-reboot-on-crash feature, so the next time of of those occurs, I should get a legit BugCheck, but I've not had one since to use for troubleshooting.
Image of what I see when this occurs (apologies for the poor quality, taking photos of a TV with a camera phone)
OS: Windows 7
CPU: Intel Pentium G645 2.9GHz
Mobo: H77MU3 LGA 1155 mATX
PSU: OCZ StealthXStream OCZ600SXS 600W
RAM: Ballistix Sport 4GB DDR3-1600
GPU: SAPPHIRE 100355OCL Radeon HD 7850 2GB 256-bit GDDR5 (OC version)
OS SSD: SSDNow V200 SV200S37A/64G 64GB SATA
Media HDD: Barracuda 2TB 7,200 RPM SATA
some SATA DVD burner
some cheap media card reader
some cheap USB3.0 hub that slots into PCI
All hardware is factory-default, no overclocking or other mods done.
Steps already taken:
Thorough dust cleaning with compressed air.
Memtest86+ on each stick of RAM individually. Both passed.
My suspicion is either PSU or GPU, both should be pretty damn healthy, as neither are a year old (both bought in April of this year). How would I test them for faults, especially given the anomalous nature of the error? This is eating away at the Spouse Acceptance Factor of my hand-rolled HTPC, so any tips on getting the stability back on track would be great.