Installed Ubuntu on partition of HD with Vista but doesn't boot

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http://askubuntu.com – after trying Ubuntu from Live CD, I've decided to install it: I partitioned the HD where my Vista installation is by shrinking it on Vista, and I've left the space empty without formatting it; then I've used the live CD to install Ubuntu (12.04), which seemed to complete fine. At boot no GRUB requester happens and Vista boots regularly, whereas from its disk management tool the empty space I had left (16 GB) has been occupied by the Ubuntu partition and by the swap' one. With that there are 4 primary partitions, albeit there was also one more I forgot to unmount before installing (a virtual (HowTos)