I suspect my desktop's motherboard has simply shit the bed. Frequently get whole system lockups, especially in early boot phase and the video card isn't even recognized in one of the two PCI slots. Evidently adding/removing USB devices exacerbates the problem.
So either:
1/ The south bridge is on its last legs, and I'm out $$$ to replace the whole board.
2/ Window has lost its damned mind and I need to reinstall.
Option 2 would be cheaper and quicker to validate so I'm trying it now. Wish me luck.