The project above continues despite some snags, but that's not the victory.
The snags I mention meant I had to go grab some utilities I bought a while ago.
Bad news, they're on my dual-PIII system. The PSU on that system died in 2011. It's a hyper-specific PSU (still has AT connectors!) so I can't just swap it out BUT it was still under warranty, so I mailed it back and had it fixed.
And then it sat there, because I was too lazy to reinstall it, even through two moves.
[...time passes...]
Fast forward to two days ago, and needing my utility collection.
Finally reinstalled that PSU.
Computer booted first try. Didn't even have to reset the date & time. Hello, good ol' WinXP!
It powered off unexpectedly 10min in, but came back up and has been stable since, so I suspect a family of dust bunnies may have moved in during the 8yrs it sat inactive. Part of the reason I retired the system was because the GPU (a Quadro FX 4000--yes, an AGP Quadro!) was getting flaky. So I bought two more (ages ago), but they also had intermittent issues. So I'm not actually sure if the shutdown wasn't due to the GPU. We'll see.
tl;dr: Computer sat in a basement for ~8 years, powered right back up on first try.
--Patrick