From: "Geo." 

http://www.theregister.co.uk/2006/08/02/windows_vista_upgrades/

[...]

This is the reality facing large number of business users still running
Windows 2000 - 56 per cent, according to latest research. Microsoft said
customers running Windows 2000 Professional are eligible for an upgrade
copy to a corresponding or better edition of Windows Vista, but a
"clean install" is still required.

[...]

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