Reinstalling from LiveCD
https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindowsIf you cannot boot from GRUB 2 review the section Boot Problems and Rescue Mode. If a reinstall becomes necessary follow these instructions. Two methods are presented; both require booting from a LiveCD (Ubuntu 9.10, Karmic Koala or later version). If the first method does not work, follow the second method, which is more complex and contains more options and instructions.