If it's 12 years old it might have one of those big cartridge processors installed. You could easily pull that out while running.
I once had a really sweet P2B-D setup with dual Coppermine P3s on it. The Athlon came out and I was hot to get on board, so this nice guy Twofer from these forums bought my old rig off of me.
Some months later, I was chatting with him. He'd been using the PC and it seemed sluggish. He checked task manager and only one CPU was showing. The other had come unsnapped at the top and partially lifted out of the slot.
He shut down, snapped it back in, fired up, and all was well.
Killing this machine may be more work than you'd think. They were built much more robustly, with wider tolerances, back in the 90s and early 2000s.
Personally, I'd take it somewhere well-ventilated and fire-resistant, outside, and run an extension cord to it. Fire it up, make sure it's running well, then splash some lighter fluid around inside and torch it up. I'd take bets if the thermal protection in the CPU would turn the machine off before something cooked to death or popped. Could be any of those.