Outlook 2003 can't be opened

Problem: Error message while opening Outlook 2003; "Cannot start Microsoft Office Outlook. Unable to open the Outlook window. The set of folders could not be opened."

Outlook shows only this message, Outlook cannot be opened.
Via Control panel - Mail - find PST options for add and remove PST:
Problem was resolved by removing one PST (with its location in default folder) - it was cut/paste to another partition on disk. Outlook worked with that pst file added back again - from that new location. User had many pst files, all other pst were on