You either have a bad hard drive, malware damage, or a bad motherboard. Middle one's easy enough to test for if things are working. Most hard drive manufacturer's have a testing program on their websites, and if those both come out fine, you probably have a bad IDE or SATA controller on your motherboard. The first two are significantly more likely though.
I got this message from a used computer that I got recently. I replaced the battery, and hard drive. It worked find until after I installed several microsoft updates and then just went blank (black screen with "operating system not found". I turned it off and the next day powered it on and it worked fine. After using it for approx. 15 minutes I tried to download an update and it rebooted again and said "operating system not fount". What gives?