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When your system boots, it reads the first sector from the hard disk, called the master boot record (MBR), and runs the code contained in that sector. The MBR code then reads the partition table (also contained in the MBR) to determine which partition is bootable and where it starts. Then it loads the first sector of the bootable partition, called the volume boot record (VBR), which contains the operating systemspecific boot code, but before executing the VBR, the MBR checks to be sure the VBR ends with the signature bytes 55AAh. The MBR will display the “Missing operating system” message if it finds that the first sector of the bootable partition (the VBR) does not end in 55AAh.

Several things can cause this to occur:

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The drive parameters entered in the BIOS Setup are incorrect or corrupted. These are the parameters defining your drive that you entered in the BIOS Setup, and they are stored in a CMOS RAM chip powered by a battery on your motherboard. Incorrect parameters will cause the MBR program to translate differently and read the wrong VBR sector, thus displaying the “Missing operating system” message. A dead CMOS battery can also cause this because it will lose or corrupt the stored drive translation and transfer mode parameters. In fact, in my experience a dead battery is one of the more likely causes. To repair this problem, check and/or replace the CMOS battery, run the BIOS Setup, go to the hard drive parameter screen, and enter the correct drive parameters. Note that most drive parameters should be set to auto or auto-detect.
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The drive is not yet partitioned and formatted on this system. This is a normal error if you try to boot the system from the hard disk before the OS installation is complete (such as if you replace the laptop’s original hard disk with a larger model or if you have removed the original partitions with FDISK). Boot to an OS startup disk (floppy or CD) and run the Setup program, which will prompt you through the partitioning and formatting process during the OS installation.
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The MBR and/or partition tables are corrupted. This can be caused by boot sector viruses, among other things. To repair with Windows 9x, cold boot (power off, then on) the system from a known noninfected, write-protected floppy or bootable CD containing the FDISK program (preferably Windows 98 or later). Enter FDISK /MBR at the command prompt, which will recopy the MBR code but not alter the partition table. Reboot.

With Windows 2000/XP, select the Recovery Console option at startup (you might need the original Windows 2000 or XP CD-ROM) and run FIXMBR, followed by FIXBOOT. Reboot the system.

If the message still persists and you need to recover the data on the drive, you will then either need to rebuild the partition tables from scratch using a third-party utility such as the DISKEDIT program included with the Symantec Norton Utilities, or hire a data-recovery specialist who can do this for you. If you don’t need to recover the data on the drive, simply reinstall the OS from scratch, which will prompt you through partitioning and formatting the drive.
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The VBR is corrupted. To repair with Windows 95/98/Me, secure a bootable floppy created by the same OS version on the hard disk that contains the SYS command from that OS. Run SYS C:, which will recopy a good VBR and system files to the volume. For Windows NT/2000/XP, you can use the Recovery Console or DiskProbe utility (found on the bootable operating system CD).
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You have a nonbootable floppy or CD in the system. Remove any floppy disks or CDs present in the laptop and reboot the system.