Little strikes fear into the heart of a system administrator like booting a Windows 2000 computer and being greeted with the prompt Missing Operating System. The first conclusion people often jump to is that the OS partition has been formatted or destroyed, or the partition table itself has become corrupted.
While this is indeed possible, it is rarely the main reason for such an error. Usually what has happened is that another partition on the system has been marked as startable. This happens often on systems that are multi-boot, or that have some kind of boot manager program installed, although it can also happen if a virus is present. When the machine starts, the computer never even sees the Windows boot loader; it tries instead to find boot information on a partition that may not contain any, and then produces this error. (In short, this error is from the BIOS itself, not from the Windows boot loader, so the problem is not with Windows per se.)
There are several ways to fix this. One is to use the Recovery Console and run the FIXBOOT command, which will repair the boot sector for the main OS partition, and also mark that partition as startable. (You may also need to run the FIXMBR command to repair the master boot record as well, if that has been modified.) Another is to use the Windows 98 FDISK utility, on a bootable floppy, to mark the proper partition, but this should really only be used as a last-ditch manuever. (Also, if you do use the FDISK tool, be sure to enable large disk support, otherwise it may not work. If you are dealing with a volume larger than 137 GB, it may not work -- you may need to create a bootable Windows 2000 CD that has been slipstreamed with Service Pack 3 to allow the Recovery Console to see the drives correctly.)
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.