The PostOffice is in a directory called WGPO.

To move it:

Move it (and all the subdirectories) to a new location which is shared with "Full Control".
Edit: HKEY_CURRENT_USER\Software\Microsoft\Mail\Microsoft Mail
Double click ServerPath and change it to the new WGPO location.
If you have WFWG or W95 users, change the ServerPath= in their MSMAIL.INI.

To start over(delete it):

Delete the WGPO directory
Edit: HKEY_CURRENT_USER\Software\Microsoft\Mail
and delete the Microsoft Mail Sub Key.