configure your bios: optical drive as 1st boot.
1. insert disk installer for your OS.
2. restart.
3. let it boot from optical drive.
4. try to check if there is any option that allows you to "REPAIR" your OS, i mean instead of doing a clean installation there should be somewhere in there that allows to click the REPAIR button. i havent tried using it but i've seen it several time whenever i'm doin a clean install of my OS.
5. pray that you'll find it.