What happens when you open Outlook itself? Is it not associated with .pst file somewhere? If you install Outlook on a C: drive, the default location for your .pst files is C:\Documents and Settings\user_id\Local Settings\Application Data\Microsoft\Outlook. But note: I think I'm right in saying that, if the email was originally sent by, or received by, Outlook, you'll never find it as a separate entity - the only way to get to it is to open Outlook and associate it (if it's not associate already) with the .pst file where you think the email is. The .pst file is typically just one very large file that only Outlook can make sense of.
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