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It is all part of the "denial of alternatives" attitude that Microsoft has had for years.
They do not want the general public to think of Linux/BSD/Solaris as an alternative to Windows, so, they deny their existence.
By making sure the demo can only be viewed by Windows users, it maintains the idea in Joe Publics head that the computer IS Windows......
Also, Microsoft need to target XP users, as that is where the majority of uptake of Vista will come from.
Can you honestly show me 1 Linux user that is even tempted to switch to Vista ?