When you save a file in xl, it saves it as a temporary name (a funny 8
character name).  If everything goes ok, xl will delete the original name and
then rename that funny name to the original's name.

But if something goes wrong, then excel gives you that message.

Typical things that can go wrong are interferance from antivirus software and
network errors (permissions or even hardware errors).

Can you disable the antivirus temporarily and see if you can eliminate that from
the list of possible causes?  Or save to a local harddrive?

(kind a divide and conquer approach.)