Why This Message Occurs

• The message means that, after the computer POST (power on system test), the computer cannot find an operating system. Finding an operating system will depend on which operating system is installed. If it is a Windows XP system it is looking for NTDETECT.COM, BOOT.INI and an assortment of files that determine what happens when Windows boots. If it is a LINUX system, it will be looking for a bootloader like LILO (Linux Loader) or GRUB (GRand Unified Bootloader), which passes control to the LINUX kernel. Then the system starts up with the user's choice of startup programs that form the desktop applications. This process is the same for both Linux and Windows.
  The term "boot" comes from the idea of "pulling the bootstraps on" the system---in computer parlance, a way of describing the system startup. This terminology stuck after first being coined by computer engineers during the beginning days of computer systems.
  
  

What the Boot Process Involves

• After the POST, the system searches for a boot device; commonly, a bootable partition on a hard disk, a CD-ROM or floppy disk, in that order nearly every time. It can also be a network device or a USB or flash memory card. Many of Cisco's internetwork devices are started from cards plugged into the front of the machine. So, if one of these devices fail, the same message, "Insert bootable media," will display on the booting screen of whatever machine is attempting to start. If the hard drive loses its partition information, this is one of the scenarios leading to this message. If the files used for startup are corrupted in any way, this is another reason for the message. The way of correcting this is to complete the system startup with an alternative boot device (See Resources), and, once booted, search for reasons why the system failed to boot from the normal boot device (in most cases the hard drive). Some of these resources also have tools to rescue booting files that are missing or bad. Another reason could be that the user left a disk in a drive and the machine is set (in the BIOS) to boot from that media first. This could be a floppy disk or a CD/DVD ROM. And the reason it does not work is because the floppy or CD/DVD does not contain a boot record. The simple remedy is to remove the floppy, CD or DVD and start again.
  
  

How Windows XP Loads Itself

• NTLDR is the loader for the Windows NT operating system. After it has accessed the file system, which can either be NTFS or FAT32, it looks for the presence of a file called HIBERFIL.SYS which is used if the user hibernated the PC. If so, NTLDR loads from HIBERFIL.SYS and the system comes back up from where it left off at the end of the previous session. Otherwise, it looks directly at BOOT.INI for information where to boot (including non-Windows NT operating systems like Linux or MS-DOS). Next, if a Windows NT operating system is selected, it runs NTDETECT.COM, which loads information about the hardware. Finally, it passes control to NTOSKRNL.EXE, sending to it the information returned by NTDETECT.COM.
  
  

Non-Booting Scenarios and Ways to Avoid Them

• There are other reasons why a machine will not boot. One of them is a corrupted system registry. This will generally not give the same error message. However, a direct reason for corrupted system startup files is the lack of space on the primary hard drive. So it is a good idea not to let the hard drive's free space fall below 25 percent. Another reason would be the beginning of failure of the hard drive's physical sectors, causing a failure of the system files needed to boot the drive. Missing any of the critical files required to boot Windows is like having a missing piece of a jigsaw puzzle.