After upgrading to Windows 7, my VM would not boot past bios -- halting immediately after doing local media detection. The only post I saw referenced an XP BSOD issue, and involved changing the number of virutal CPU's to 1, from 2. (This didn't apply to me, as mine was already set to 1) I changed it to two, and then immediately back to 1. I then disabled Hardware/ Hypervisor Acceleration, and then was able to boot my instance. I hope this helps you. I didn't make any changes. But the VM definitely was unable to boot before, and did after doing above.
