There may be simpler methods, but one that worked for me before I moved to hosted exchange for outlook was as follows:
1) Back up the data file.
2) Delete Outlook thoroughly -- removed through control panel, all registry entries, and so on -- search for .pst files and kill them.
3) Re-install Outlook. Create a new profile.
4) Restore the backed up file.
5) Open the backed up file.
6) Move the data to the new file.
(Before 2010, a simpler procedure existed, and it may still, with non-exchange installs. If there is no profile, or if you create the new one then delete it, Outlook would allow you to search for a data file. I don't know if that still works.)