Comment 39 for bug 477104

If you have "ALERT! /hosts/ubuntu/disks/root.disk does not exist." it is because when you run the command

linux /boot/vmlinuz-2.6.31-14-generic root=/dev/sda1 loop=/ubuntu/disks/root.disk ro quiet splash

you need to replace /dev/sda1 with the appropriate partition, /dev/sda1 means that wubi is to be found on the first partition of the first disk, which might not be the case.

If you have a corrupted /boot/grub/grub.cfg, please try to boot manually following the instructions above, do a system upgrade, the regenerate the grub configuration with the command: sudo update-grub

Then check whether the file /boot/grub.cfg is meaningful or just a bunch of random chars
If it seems ok try to reboot, otherwise please report it here.