After the POST, computers must find and load the Windows Boot Manager. Older BIOS computers and newer EFI computers do this slightly differently.

Initial Startup Phase for BIOS Computers

After the POST, the settings that are stored in the nonvolatile memory, such as boot order, determine the devices that the computer can use to start an operating system. In addition to floppy disks or hard disks attached to Advanced Technology Attachment (ATA), Serial ATA, and small computer system interface (SCSI) controllers, computers can typically start an operating system from other devices, such as the following:

• CDs or DVDs
• Network adapters
• Universal serial bus (USB) flash drives
• Removable disks
• Secondary storage devices installed in docking stations for portable computers

It is possible to specify a custom boot order, such as CDROM, Floppy, Hard Disk. When you specify CDROM, Floppy, Hard Disk as a boot order, the following events occur at startup:

1. The computer searches the CD-ROM for bootable media. If a bootable CD or DVD is present, the computer uses the media as the startup device. Otherwise, the computer searches the next device in the boot order. You cannot use a non-bootable CD or DVD to start your system. The presence of a non-bootable CD or DVD in the CD-ROM drive can add to the time the system requires to start. If you do not intend to start the computer from CD, remove all CDs from the CD-ROM drive before restarting.
2. The computer searches the floppy disk for bootable media. If a bootable floppy is present, the computer uses the floppy disk as the startup device and loads the first sector (sector 0, the floppy disk boot sector) into memory. Otherwise, the computer searches the next device in the boot order or displays an error message.
3. The computer uses the hard disk as the startup device. The computer typically uses the hard disk as the startup device only when the CD-ROM drive and the floppy disk drive are empty.

There are exceptions in which code on bootable media transfers control to the hard disk. For example, when you start your system by using the bootable Windows DVD, Windows Setup checks the hard disk for Windows installations. If one is found, you have the option of bypassing DVD startup by not responding to the Press Any Key To Boot From CD Or DVD prompt that appears. This prompt is actually displayed by the startup program located on the Windows DVD, not by your computer's hardware.

If startup fails during the initial startup phase, you are experiencing a problem with the BIOS configuration, the disk subsystem, or the file system. The following error message is common during this phase. It indicates that none of the configured bootable media types was available.

Non-system disk or disk error
Replace and press any key when ready

If you changed the disk configuration recently, verify that all cables are properly connected and jumpers are correctly configured. If booting from the hard disk, verify that all removable media have been removed. If booting from a CD or DVD, verify that the BIOS is configured to start from the CD or DVD and that the Windows medium is present. If the disk subsystem and BIOS are configured correctly, the problem may be related to the file system.

If you boot from the hard disk, the computer reads the boot code instructions located on the MBR. The MBR is the first sector of data on the startup hard disk. The MBR contains instructions (called boot code) and a table (called a partition table) that identify primary and extended partitions. The BIOS reads the MBR into memory and transfers control to the code in the MBR.

The computer then searches the partition table for the active partition, also known as a bootable partition. The first sector of the active partition contains boot code that enables the computer to do the following:

• Read the contents of the file system used.
• Locate and start a 16-bit stub program (Bootmgr) in the root directory of the boot volume. This stub program switches the processor into 32- or 64-bit Protected mode and loads the 32- or 64-bit Windows Boot Manager, which is stored in the same Bootmgr file. After the Windows Boot Manager loads, startup is identical for both BIOS and EFI computers.

If an active partition does not exist or if boot sector information is missing or corrupt, a message similar to any of the following might appear:

• Invalid partition table
• Error loading operating system
• Missing operating system

If an active partition is successfully located, the code in the boot sector locates and starts Windows Boot Loader (WinLoad) and the BIOS transfers execution to it.

Initial Startup Phase for EFI Computers

If you install a different operating system or manually change the EFI boot manager settings, EFI might no longer load the Windows Boot Manager. To resolve this problem, use the Startup Repair tool. Alternatively, you might be able to update the EFI boot manager settings manually using your computer's built-in EFI tools.