In a dual boot system (Windows & Ubuntu), the straight forward strategy should be, first you install windows and then ubuntu. If you install ubuntu prior to windows, system cannot boot into ubuntu because windows bootloader (NTLDR) cannot boot Linux what Linux bootloader (GRUB) can. At this situation you need to reinstall GRUB.
Follow the steps,
1. Insert your Ubuntu live CD into your computer and restart it.Boot from it (Try ubuntu).
2. Open terminal and type
sudo fdisk -l
and locate which partition contains your linux installation(e.g /dev/sda9).
3. Then type
sudo mkdir /media/root
sudo mount /dev/sda9 /media/root
(sda9 should be replaced by your linux partition)
If you’ve done this correctly, then you should see the following output if you type ls /media/root
bin dev home lib mnt root srv usr boot etc initrd lib64 opt sbin sys var cdrom initrd.img media proc selinux tmp vmlinuz
4. At last type
sudo grub-install –root-directory=/media/root /dev/sda
after successful execution the output will come “Installation finished. No error reported.” If it comes,then you can reboot your computer.
If the output doesn’t come like this,type
sudo grub-install –root-directory=/media/root /dev/sda -f
(that is with the force option)