Using Multiple Operating Systems

• When more than one operating system is installed on a single computer, the computer requires the use of a program to allow selection between them. XP and earlier versions of Windows used a configuration file that was scanned at startup. Windows NT used a program called ntldr, which passed the boot information to BIOS. Microsoft Vista uses a small system file at startup called bootmgr. They all serve the purpose of identifying available operating systems and providing a way for the user to choose, usually in the form of a simple menu.
  
  

Vista and Bootmgr

• When the computer starts, Vista passes control to bootmgr, which then loads a file called BCD, short for Boot Configuration Data. Bootmgr used the information contained in that file to create a menu and then waits a preset period of time for the user to make a choice. After the choice is made, control is passed to the boot record for the operating system that was selected. If no selection is made, control is passed to the default OS.
  
  

XP Boot Files

• Windows XP uses a file called boot.ini to store information about available operating systems. To work properly, the program ntldr must be installed as well. Ntldr scans the boot.ini file and creates a user menu with a delay timer. if no selection is made before the timer expires, the default OS will be loaded. XP does not use the bootmgr program, and the XP and Vista startup systems are not directly compatible.
  
  

Avoiding Bootup Errors

• All Windows versions are built to be backward-compatible. This means that when you want to install multiple versions, such as XP and Vista, you need to install the newer OS first. In this instance, install Vista and then install XP after Vista is successfully installed and boots up. This allows the BCD to be properly updated by bootmgr so that all available operating systems are properly cataloged.