When a computer starts up it searches various devices for an operating system (like Windows).
The three main places it looks are the hard disks, CD drive, or diskette drive.
If the PC cannot find the Windows operating system you get a message like the one you are getting.
So it either means the operating system is MISSING, or the PC is not looking in the right place.
One of the things you can set in the BIOS of a PC is WHERE to look for an operating system, and in what ORDER.
It sounds to me as though someone has changed the BIOS so it no longer looks for an operating system on the hard disk.
You need to go into the BIOS of your PC and look for an option like BOOT ORDER or BOOT DEVICES or something like that.
In there make sure the HARD DISK is in the list of devices that the PC looks for an operating system.
The order is not that important, but probably CD drive, then hard disk, then diskette is a good default order.
You go into the BIOS by pressing a button BEFORE you go into Windows. You should briefly get a message on the screen saying what button to press to go into the BIOS.
You MUST press it BEFORE Windows starts.
Good luck.