Problem in dual boot with Ubuntu 12.04 and windows 8 (can't choose an os when boot)

In my laptop initially it was Windows 7 and 8 to dual boot.The status of the partitions are "system" for win 7 and "boot" for win 8. Then I formatted win 7 which was the system partition and installed ubuntu 12.04 with windows 8 (I chose install ubuntu alongside with windows 8). Now it doesn't appear any list to select an operating system instead it directly loads ubuntu.

How to solve this?

I would suggest trying to repair GRUB. The easiest way to do that is probably to use Boot Repair application.

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair

After installation launch it using Bash and try default repair. If it will not help You can try advanced options.

And if anything goes wrong remember that You can also use this application with Live-CD.

Hope that will help.

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I just get a an idea. You wrote that You cannot see any boot menu. Try to check grub with commend below.

sudo gedit /etc/default/grub

Check out this line:

GRUB_TIMEOUT=

. What number is there? If there is 0 You should change it to higher number, save file and update grub with

sudo update-grub

as I think.
But anyway the easiest and safest way is to use Boot Repair application. It changes this line by default also as I know.