Totally uinstalled MSE and installed Avast! yesterday. The problem keeps occurring, only this time there's no events in the Event Viewer OR blue screens.
It happens once or twice in a row, then Windows will start successfully. I can't give you a real ratio, because once the computer boots, I start using it -- so I only really notice the fails, if you know what I mean.
The computer seems to work perfectly ONCE Windows starts -- it's only the Starting Windows part that freezes.
I haven't really tried just turning it on and off in Safe Mode, either... it's largely just a question of having the time to turn the computer on and off repeatedly to check. But I'll give it a few tries now.