Operating System not found

Here is a strange one, Installed Windows XP Prof. using a Promise Ultra 66 controller card on a WDC 18gb drive, this is an older Micron PC 500MHz 256 2 DIMMS. Bios has been flashed all Updates to XP have been installed. When I boot EVERY OTHER BOOT comes up with “Operating System not found”. I reboot and everything is fine. But the next time I reboot, it will do the same thing again “Operating System not found”

? ? ? ? help ? ?