Where is this calendar folder located?  His personal folders?  His Exchange
mailbox?  A shared mailbox?  A delegate's mailbox?  A public folder mailbox?

My Refusal to Save Problem Solved.

Your calendar data is stored in a subdirectory called Outlook.PST.  On my
computer the (needlessly long default) directory tree for the Outlook data
file is:  C:\Documents and Settings\HP_Owner\Local Settings\Application
Data\Microsoft\Outlook\Outlook.pst

[It does not help that Outlook.pst is a hidden file and turning off the
hidden file block filter in my Windows XP search still did not result in hit
with the Windows XP search function.]

I have a HP Pavilion computer that is a few months old on which the Outlook
2003 trial period was expiring -- along with the remainder of the Office 2003
suite.  I bought a stand alone Outlook 2003 program on a CD and ran install
which automatically asked if I wanted to delete my old version, which I  did.
Voila, the old calendar data was there without needing to copy the old
Outlook. pst data file (that I had saved in an unrelated directory) into the
proper subdirectory.  However, when I tried to save a new entry into my
calendar I got the following error message:

“The item could not be saved to this folder.  The folder has been deleted or
moved, or you do not have permission.  Do you want to save a copy of it to
the default folder for the item?”

When I choose “save to the default folder . . .” I get another error message
which read:  “The message interface has returned an unknown error.  If the
problem persists, restart Outlook.”

Hint, the error messages had nothing to do with the real problem -- or at
least nothing apparent to a non-IT professional.

Restarting Outlook did not help.  Reinstalling Outlook did not help.  
Uninstalling Outlook and reinstalling Outlook did not help.  Deleting the
file Outlook.PST and uninstalling Outlook and reinstalling Outlook without
the former PST file resulted in Outlook not running.  Deleting the file
Outlook.PST and uninstalling Outlook and  and copying the old PST file back
to the proper location resulted in the same problem all over again.

Hours later:  I uninstalled Outlook 2003, using the Windows uninstall rather
than the "auto" unintall on the new disk.  I deleted the Outlook.PST file.  
Then (here is the key) I uninstalled the entire trial version of Office
(which was expiring in a few days and I am a Word Perfect user anyway).  I
reinstalled the new Outlook 2003 and copied the old Outlook.PST data file
into the slot at the end of that (needlessly long) directory tree.  Now my
Outlook calendar saves new entries and I have my old entries.  

Some gremlin was being left behind somewhere in the bowels of the remainder
of the trial version of Office 2003 whenever I separately uninstalled Outlook
2003, then it would relink to my new Outlook 2003 as soon as I reinstalled
it.  Good Grief!