adding a line to boot.ini will not fix things
you will also need winload.exe copied from vista sys32 to XP sys32
HOWEVER
There is a "new" Recov console in Vista - WinRE
http://blogs.msdn.com/winre/
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As mentioned in the first post on WinRE, the recovery console has been deprecated in Windows Vista. So you ask what happened to all those wonderful commands that were available in recovery console. Well, we were kind of hoping that you wouldn't need them anymore. But if you do, you'll be glad to know that most of them are available via the command line in WinRE. Here's a list of recovery console commands that are different or unavailable in WinRE:
etc.
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There is also a startup repair option in "repair your computer" (after you pick Vista install in window)
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http://blogs.msdn.com/winre/archive/2006/09/18/760295.aspx#760350
How to get to setup repair^
How can end-users get access to WinRE and Startup Repair?
For Vista RC1 release, WinRE is available on the Windows installation DVD. To access WinRE, boot using the installation DVD, choose your language settings and hit "Next" and then click on "Repair Your Computer" link at the bottom of the screen. This will launch WinRE. After choosing an OS to repair, you should see all the recovery tools available in WinRE.
After Windows Vista releases, WinRE will be available via other convenient means as well. For example, OEMs may pre-install WinRE on the hard disk and corporations (SA) can make WinRE available for network boot via a WDS server.
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This will prob fix your issues
I would really be appreciative if you would first boot to your XP CD and go its recov console and in admin mode type in BOOTCFG /REBUILD (for "C" volume") (space after G)
I am really curious as to whether XP will change the bootini to reflect Vista, or whether it puts in usenewloader
I also note that your post was moved without any notice from GH so you might not come back. If you do, I will enable my PM and you can tell me the outcome