I would guess that holding ALT on bootup is giving you the install in the MBR... One of the tux icons should now.... at least one...

PS, at the grub menu, you can use the grub console to load Ubuntu... I know this is not a solution, but it might garner more info.

Code:
grub> root (hd0,2)
grub> kernel /boot/vmlinuz-2.6.24-19-generic
grub> initrd /boot/initrd.img-2.6.24-19-generic
grub> boot