Restore GRUB2 after installing Windows
How I Restored GRUB2 after a Windows 7 Install. GRUB2 (Grand Unified Bootloader) is a universal bootloader used by Ubuntu and other Linux Distributions. If you have a Linux Distribution installed on a partition, and attempt to install Windows alongside it, your GRUB bootloader will be overwritten by the Windows bootloader, and your computer will only boot into Windows.
Recovering the GRUB2 Bootloader after a Windows Install is simple. When finished, your Bootloader should detect all installed Operating Systems and automatically add a bootable menu item for each of them.