What version of Outlook?
I spent ages trying to get Outlook 2000 to work on my wife's new Vista laptop, but it can't be done. Apparently there is a fundamental incompatibility.
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Why do you want to run Outlook instead of Windows Mail? If you use Outlook for its calendar functions, task reminders, etc. you'll find that all of those exist within Vista itself.
So, having given up on Outlook 2000, we found that actually Vista/Windows Mail would do everything that used to use Outlook for.