Sami did some DLL hacking and got Avalon running on Windows XP... I missed the post, but someone internally forwarded me that he was asking what I thought of this. I guess there are a couple points here:

Avalon is a Longhorn only feature.

One of the big new changes that isn't yet in the Longhorn builds is the new video driver model. Once that goes in, you really won't be able to run Avalon on XP. In addition when the desktop composition stuff gets turned on (it wasn't in the PDC handout build) there will be a tight dependency between Avalon and the Longhorn User32.

As with any single feature in an operating system, you can manage to get it running elsewhere. If you really wanted you could probably copy the Longhorn Kernel32 over to an XP machine, however eventually you will need to copy most of the OS over to XP to make all the features work.

Early on in the Avalon development we actually ran Avalon on XP all the time (there was no real "Longhorn" yet). For more than a year now Avalon was only running on Longhorn. Now that we are pushing towards Beta 1 the number of inter-dependencies between components are increasing.

In short, while you may have been able to get this working on the PDC build, this won't be possible in the future builds.

I guess my advice to Sami, I wouldn't bother publishing the steps needed for this. Not only is it a completely unsupported configuration, it is also something that won't work going forward.