MemTest 86+ started reporting errors immediately. In fact, after letting it go through 4 complete tests it was reporting that only 8% of results were good. AT least that's how I interpreted it - the top said PASS: 8%.

The bottom of the screen was perpetually RED (error/fail).

I'm bringing the system upstairs now to start pulling components. I don't understand how the memory could suddenly go bad in a system that runs cool all the time. Not only that, but if the memory is truly as faulty as reported, how is it that the Windows installer loads at all?

I would have a much easier time believing it's the motherboard and memory controller that have gone to crap. More finger pointing at ASUS.

Prior to running MemTest, the Windows install failed again. This time after confirming some drivers (that were already on the system and have been there since day 1). The blue screen error this time was "irql_not_less_or_equal"