An hour and a half later, I tried again and lo and behold I went past the Asus screen and got a message from AMI saying "Overclocking failed". I went through every single item in the BIOS. Nothing had changed since my first successful installation a year ago. Overclocking was still at "standard" (I never did try to fire things up over the past year).
From then on, everything worked fine.
Is there an explanation somebody could provide?