If you have installed, for example, XP, on a Vista machine, in the Boot Menu, you may no longer be able to see Vista as an option. This occurs because when a different version of Windows is installed, it rewrites the MBR to call its own boot loader. Let's see how to recover it.
So if you install Vista on a PC where XP is already installed,
the Vista boot menu incorporates the options from the older OS's boot
menu.
But, on the other hand, if you install XP on Vista, it will
overwrite the MBR with one that does not recognize the newer Vista's
Boot Loader.
To resolve this issue, open cmd in XP.
Run C:\Boot\ Bootsect.exe –NT60 All
Where C is the Drive letter.
Reboot.
You will now be able to see the Vista menu option/s.
Now to restore the entry for XP, open an elevated command prompt and enter this:
Bcdedit –create {ntldr} –d “Put Description of Menu here”
Restart Computer.
