Ubuntu installation on a system which already has other operating systems is simple. Ubuntu will automatically detect other operating systems already installed on the computer and enable booting into them from the grub boot menu. However if you install Windows on a system which already has Ubuntu installed on a partition other than the partition on to which you install windows then you will inevitably lose the option to boot into the Ubuntu installation. There is a simple way to rectify this issue.
Boot into Ubuntu using the Ubuntu Live CD
Open Gedit and copy the following code into it

sudo bash
mkdir /mnt/hdd
mount /dev/<hda6> /mnt/hdd
grub-install --root-directory=/mnt/hdd /dev/<hda>

Change <hda6> to whichever partition your /boot resides
Change <hda> to whichever harddisk you want to install grub to
Open a terminal, copy the code from GEdit and paste into the terminal
Presto! You are done
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