After installaing a new OS on an additional Partition ist this the only thing to add it on the Menu from Grub2.
Re-generate /boot/grub/grub.cfg as root
grub-mkconfig > /boot/grub/grub.cfg

If you installed Windows after the Linux, and now your grub is not showing only Window is booting.
Just follow these simple commands you will get grub on your pc.

install grub 2

First of all insert your live Linux or Ubuntu CD/USB stick and select option live run.
When live version open then open Terminal (command line) and copy the following commands the Terminal:
sudo fdisk -l
Now you need to know the partition where your Linux/Ubuntu is installed, Note the (sda_, sdb_, sdc_) number and Now enter the following commands: (replace your drive number with sda1 in the following command.)

sudo mount /dev/sda1 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc

After this Now enter the following commands:

sudo chroot /mnt

For install grub enter the following command:

sudo apt-get install grub-pc
sudo apt-get install-grub2

Now update grub

sudo update-grub2
sudo update-grub

That’s it.
Reboot your pc