Let’s talk about Windows Vista Stratup process first: You can install Windows XP over Windows Vista. You need to perform few steps before you can get both the OS running. Earlier versions of Windows used to parse BOOT.INI to load Operating Systems. Windows XP and earlier NT-line technologies use NTLDR as their boot loader. In Windows Vista, it has been changed and now Winload.exe is the boot loader for all the operating systems. This exe is invoked by Windows Boot Manager (Bootmgr.exe) Windows Boot Manager reads the Boot Configuration Data and displays and operating system list. Boot Configuration Data replaces the BOOT.INI used in previous versions of Windows. Boot Configuration Data is stored in a data file in C:\Boot\BCD. Boot Configuration Data can be altered by a command line tool (bcedit.exe) or using WMI script. The Boot Configuration Data includes the following entries: Boot Windows Vista – (using Winload.exe)
Resume Windows Vista – (using Winresume.exe)
Boot earlier Versions of Windows – (using NTLDR)
Why Microsoft has to replace BOOT.INI and NTLDR?
The previous versions of Windows weren’t capable enough to support API call. Microsoft has changed this in coming versions of Windows. Third party vendors now can develop an application and implement the same with Vista Startup process.
Ummm…I’m focusing more on Windows Vista Startup process rather than preserving focus on main title of this article. Anyway…the below article can help you get your Windows Vista and XP running back. Remember: Windows Vista doesn’t support NTLDR and BOOT.INI but understand them and Windows XP doesn’t understand Winload.exe and Boot Configuration Data but is manageable through Windows Boot Manager. You only need to configure the startup part using Bootsect.exe of Windows Vista. Check out here:
Windows Vista no longer starts after you install an earlier version of the Windows operating system in a dual-boot configuration
Will sought more on this if I could extrapolate few more to fill in……..