Here's a little public service (and a reference point for me the next
time this happens if I forget) for people stuck with this type of Excel
behaviour...

You try to save a file (for me, 53Mb excel file using extensive VBA and
autofiltering to an NTFS compressed network share) and get the
following type of message;

Your changes could not be saved to '<<filename.xls", but were
saved to a temporary document named 'D933D120'. Close the
existing document, then open the temporary document and
save it under a new name."

You attempt to save and get an error "File not saved".

You close, reboot, wait for a three-quarter moon etc etc, try to open
the temporary file and get the error
"Excel cannot access 'D933D120'. The document may be read-only or
encrypted."

Thank you to those authors describing the above as a garbage bucket
error message, commonly having nothing to do with either read-only
attributes or encryption, which got me thinking "what else could stop a
file being opened".

Here's the solution;
Right click the file
Properties
Security

For me, the security settings were blank. It appears that the saving
process which creates this temp file when "overwriting" (actually
creates new file(the temp file above), deletes old, renames new with
old name) an old file fell over before it could apply the relevant
access lists and rename the file.

Simply add yourself as an authorised user for the file, with full
permissions.

I was then able to open the file, rename etcetera, as described on the
original error message.

As to the original cause of the issue, ie why the save process fell
over;
?http://support.microsoft.com/kb/291204/ - didn't get identical
messages, so not 100%

?Various mentions of antivirus software - unconfirmed

?NTFS compression - takes so long on some large files that a timeout of
some sort occurs- hypothetical only. May depend on network traffic at
the time.