I've looked at:
The bottom line is, after a Windows 7 install, the only way I could get the Ubuntu LiveCD (or FreeBSD, or Debian, or Ultimate Boot CD) to boot, was to unplug the SATA cable on the Windows partition (I was lucky to have more than one HDD).
The issue is purely academic now, since I used my "unplug drive" workaround last month. I've put a bounty on this because I want to know for my own benefit, and unplugging a drive is not a feasible solution. Thank you to everyone who has contributed to this question. I value your input.