There are many reasons as to why this error may occur and how to to sovle this issues but it sounds like you have tried to start a windows installaiton and becasue the disc was scratched it hasn't completed the it hence it comes up with the error "BootMgr mssing". The best solution here I think would be for you to get another Windows disc and try re-installing windows.
Does the motherbaord have a VGA port on it? is so unplug the graphics card and plug the monitor into the motherboard. Unplug all the hard drvies and optical drives. You can also remove any other RAM sticks that you may have plugged in and just leave the one in the primary slot. $Turn on PC and see if that will allow you to boot into the BIOS.
This problem sounded familiar and having read your last post I've remembered. It's quite a few years now but I decided to link my old XP computer to a 32" LCD TV with a VGA port.. It wouldn't do the job and things were as you describe until I went to the nVidia site and tried out earlier drivers for the graphics card. The setup has worked well ever since.