I've got a PC which two operating systems: Windows (XP) and Ubuntu (10.04). I had wisely installed Windows at first and after that Ubuntu, having read about Windows being more rude than Ubuntu when it comes to overwriting the master boot record.
Recently I reinstalled Windows. After rebooting, of course, Windows' boot loader had defeated Ubuntu's with the result that I couldn't see my Ubuntu during boot and thus couldn't start Ubuntu any longer...
I found a great help article about just that thing on:
The central steps which helped me out were: