IF your PC has dual boot Windows and Ubuntu, you may probably face this problem, GRUB war 
. I also faced this problem and would like to share the solution i found by Googling . . .
Reinstall Ubuntu 10.xx GRUB after Windows 7 install ***
Boot Ubuntu 10.xx(xx- Your Ubuntu version ) from CD/USB.
Open the Terminal and run the code
sudo fdisk -l
You will get a list like this.
From this list find the directory you have installed your Ubuntu. In my case it’s “/dev/sda7″
Now type the following code in your terminal
sudo mkdir /media/sda7 sudo mount /dev/sda7 /media/sda7
Finally reinstall the GRUP
sudo grub-install --root-directory=/media/sda7 /dev/sda
That’s all, Restart your PC. The home welcomes you.
Reinstall Windows 7 GRUB if you want to stay with Windows forever ***
Boot windows 7 from DVD
In the Windows installation window select “Repair Your Computer”
After it identified your Windows 7 you will get a System Recovery Window.
Select Command Prompt and run the following codes one by one.
bootrec.exe /fixboot
bootrec.exe /fixmbr
Close the windows and restart, that’s ALL.
Some screenshots of GRUB war ;)
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* I faced this problem for a long time and most of the time i failed to repair the GRUP and most of the time i pushed into a situation ,reinstalling the entire OS again and that would be my hope. Reinstalling a OS is not a big task but after a fresh install talking your System into previous environment is too difficult and consumes lots of time.
** I doubted how many people gonna try this solution but i’m 100% sure there is a one person surely gonna follow this methods, THAT’S ME . . . 
*** It works on my PC ! :)
