SoftPedia published an article about uninstalling Windows Vista and restoring the previous version of Windows

Uninstalling Windows Vista and restoring the previously deployed version of Windows – be it Windows XP, Windows Server 2003, Windows 2000 or even a low level edition of Vista – is quite a task. If you don't know your way around the command prompts, then you'd better stay away from this one or ask for help. But otherwise, there is nothing stopping you from manually removing Windows Vista and replace it with the previous Windows platform you had installed. But be advised, the process does require a "steady hand" as it is laborious and full of meanders. Microsoft has yet to automate the removal of Windows Vista and there are no signs that point to this feature with the first service pack for the operating system.

Uninstall Windows Vista and Restore the Previous Versions of Windows