Total time to install the hardware: less than 45 minutes, and that included a fair bit of arguing with the CPU fan when I tried to remove it from the old motherboard.
Total time to convince goddamned motherfucking sonofabitch XP to run? Somewhere between three and four hours.
Total number of reboots required to accomplish this feat: I lost track after about twenty. Luckily Windows did most of them on its own, without my having to touch anything. It wasn't too keen on that booting up thing. "Oh, look... it got halfway through the boot sequence and changed its mind. How nice." I never did figure out which of the drivers was making it spit up-- I just started removing old ones with extreme prejudice. I was so pissed that I was swearing in three different languages.
I really really REALLY want to find some way to tell Windows, "Dude, if you ever move even one icon on my desktop even a millimeter, let alone rearrange all of them, I will remove all of your drivers with an Epilady."
The one good thing is that it didn't seem to need to reactivate itself, like I expected it to.
The hardware part of this was trivially easy: unplug as much as you can reach, unscrew the motherboard from the case, pull it out and unplug the rest of the cables. Pull the memory and CPU, install them in the new motherboard. Plug in the problem cables from before, screw the motherboard into the case, plug the rest of the stuff in. My only mistake was the one that I make every single time I do this-- the reset button plugs into pins one and two, and the power switch plugs into four and five. I always want to plug the power switch into three and four. And then I wind up powering the machine up by shorting across pins with a screwdriver a couple of times before I remember what I screwed up.