A trick has been posted lately at Vista.Blorge.com about installing a full version of Windows Vista from an upgrade version disc.
The instruction is:
1. Install it without the license key.
2. “Upgrade” the installation and enter the license key.
Therefore, it means that all a user has to do is install the upgrade version twice and save about $100 from buying the full version.
“The fact that the company has not taken steps to remove the upgrade trick from Vista SP1 leads us to believe that Microsoft executives knowingly support the trick,” said the Windows Secrets editorial director.
Source: Vista.Blorge.com

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If you have to do two installs anyway, why not just install XP first then do the upgrade for Vista? Then, not only will save the same $100 on your purchase of Vista but will save 45 minutes to an hour on the first install this “trick” requires.
you should read Ed Bott’s take on this : “The fact that you can work around a technical limitation doesn’t automatically make the practice legal.”
http://blogs.zdnet.com/Bott/?p=420
There is no legal issues with installing XP first then using the Vista upgrade DVD to install Vista. That is exactly what the Vista Upgrade DVD is meant to do.
This is, of course, assuming that the XP license is legal and valid for the machine in question.
Good point Jm Boyd.
I disagree with the statement: ““The fact that the company has not taken steps to remove the upgrade trick from Vista SP1 leads us to believe that Microsoft executives knowingly support the trick,” said the Windows Secrets editorial director.”
Microsoft is aware of many bugs in their software, but that doesn’t mean they are speedy about fixing them
Although, it is true you would think that they would start with the ones which could really affect their bottom line.