I work with a user who has Office 2000 Standard SR-1a.  
Recently he has been sent files from other users that have
Office 2000.  Without any consistency he has not been able
to open some Excel files.  These files do not have links
or macros.  He receives a cannot open file error
or 'file.xls' cannot be accessed.  The file may be read-
only, or you may be trying to access a read-only
location.  Or, the server the document is stored on may
not be responding.

I had him uninstall Office 2000 reboot the computer and
then re-install Office.  He is still afflicted with the
same problem.  

When I have people send me the same files he was not able
to open, I have no problem opening them in Office 2000.

He has a current virus signature of Norton Anti-Virus.  A
scan of his drive reveals no viruses.

What might be causing this?  What solutions can be done to
resolve these errors?

Thank you in advance.