Have you tried already changing the 'priority' on your BIOS? just dont change anything except for that in BIOS because you might brick the system. Just change the CD/DVD drive as the first boot priority and then save and restart. Insert the installation disc of your OS and now is should boot to your disc. Do not re-install the system yet, but try to choose the 'repair' and try the tutorial i have posted above using MS-DOS (command prompt).