One of the side effects during the upgrade is to load the GRUB loader menu from which I select the OS to load. Windows overrides that upon installation so it is completely by passed.
Luckily, this is a well covered topic at ubuntu forums since it is a likely thing to occur
A nice guide on how to restore the grub menu can be found at https://help.ubuntu.com/community/Grub2#Reinstalling%20from%20LiveCD
This is useful if you are using GRUB2, ubuntu 9.10+
- Use a live CD to boot ubuntu on you computer
- mount the system partition (or the partition having the /boot directory) in the original installation of ubuntu that you have to a specific location (say /meda/ubuntu_sys)
- use the grub-install command:
sudo grub-install --root-directory=/media/ubuntu_sys /dev/sda