I have a 2003 Microsoft Small Business Server. I installed a program Symantec Endpoint Protection, to run as a centralized Anti-Virus solution. The server had to reboot to complete installation, and when this reboot happened, I was unable to boot to the drive with the OS. I am not sure if installing this is a coincidence or not, but I wanted to give all the facts of what I was doing right before the error. When I reboot, I now get the "Operating System Not Found" error. I am not runnning a RAID, and have checked my BIOS for boot priority, and the HD is set as the first boot device.