It's in the Services configuration and the service is called 'Personal
Folders'. Remove the existing Personal Folders service - this will not
delete the .PST file. Move the PST file to where you want it, then add the
'Personal Folders' service back again but when asked for the name of the PST
file, browse to the existing .PST file that you just moved.
In Outlook XP I believe this is under File -> Data File Management. In all
other version it's probably somewhere under Tools -> Services
