editing of the Boot.ini alone will not help, as Vista does not make use of the good old NTLDR, and boot.ini only affects the NTLDRs options of what and how to boot.
Vista uses "Bootmgr" which replaces the MBR code that XP has written and can be edited from within Vistas CMD, if it is started with Administrators Rights per calling of BCDEDIT.exe within CMD.
This tool is very complex in editing and if wrong used it can do more harm than help.
To make a long story short ...
Before you go into deep trouble I would suggest you a very simple to use tool called "VistaBootPro". It is free and works on RC1.
But of course there are no warranties if something goes wrong. Know that.
To add XP to Vistas Bootloader or to change vistas Boot-Entries ( if you have 2/3/4 different Vistas on the HDD ) this is it:
http://www.vistabootpro.org/
Please read how to use it and have a look in the forums there.
It is definately simpler than Vistas own tools and usually it is doing its job very good.
SBJ
