To install an older Windows operating system on a computer running Windows Vista, use the following procedure.

1. Install the previous version of Windows, e.g. XP or 2000
2. Log on to the older operating system and restore the latest boot manager by running the following.
   
   bootsect.exe -NT60 All
   
   bootsect.exe might be in your boot folder of active partition. If not there, then it's on your Vista DVD in boot folder.
3. Create a BCD entry for the older operating system by specifying the following. Bcdedit.exe is located in the \Windows\System32 directory of the Windows Vista partition. Description is the description of the new entry for the older operating system.
   
    
   Bcdedit /create {legacy} /d “Description”
   Bcdedit /set {legacy} device boot
   Bcdedit /set {legacy} path \ntldr
   Bcdedit /displayorder {legacy} /addlast
4. Restart the computer in order for the changes to take effect.

You can use a third party program like EasyBCD or VistaBootPro, but apparently, behind the screens, both programs run this command to restore the bootloader. Go to your EasyBCD installation folder and you'll find bootsect.exe.

Also, if you want to revert back to XP's bootloader, then run

           bootsect.exe -nt52

happy computing!!

source: http://www.msfn.org/board/How-to-Install-XP-after-Vista-and-Fix-th-t100672.html

yes

Mudgal ji, Pls mention the source in future posts.