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.
So if you find that Vista is missing as an option in a multi-boot computer, you can resolve this issue, as follows : Open cmd in XP.
Run
Where
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.
If this trick doesn't works then follow this:::
After installing windows XP, Boot with Vista DVD and at the first screen select to repair startup of Vista. It will repair the startup of vista but now you will not be able to log into XP. for this download a free software "EasyBCD" (Google for it). run it in vista and follow the instructions on EasyBCD's website. It will add Windows XP link on windows selection screen.
Enjoy with new coming PC's or Laptops with preinstalled Vista.
Just install both OS if you have.