This seems to be self explanatory just insert the Win 7 CD and follow the below given instructions and then enter the commands one by one as mentioned below strictly without missing anyone of them.
Select your language preferences and click on Next.
Click on Repair your computer.
Select which operating system you want to restore and the click on Next. If Windows 7 is not listed here, or it is blank, then it is ok. Click on Next anyway.
Select the system recovery option you want to do:
System Restore System Image Recovery Windows Memory Diagnostic Command Prompt
If you managed to recover Windows 7, will have lost Grub should reinstall.
Insert the Ubuntu Desktop live-disc into the DVD drive and restart the computer.
Select Try it.
Open a terminal.
sudo -i fdisk -l #Suppose fdisk informs your partition / is /dev/sda3, continue running mount /dev/sda3 /mnt mount --bind /dev /mnt/dev mount --bind /dev/pts /mnt/dev/pts mount --bind /proc /mnt/proc mount --bind /sys /mnt/sys chroot /mnt grub-mkconfig -o /boot/grub/grub.cfg grub-install --root-directory=/mnt /dev/sda grub-install --recheck /dev/sda umount /mnt reboot
via – arch wiki & Microsoft forums.