When excel saves the file, it saves it as a temporary file with a funny name (8
characters--no extension) in the same folder.
If the save is successful, xl will delete the original (or rename it to its
backup name (like "backup of book1.xlk)) and if that's successful, xl will
rename the funny named file to the original's name.
So for an instant, excel needs about twice as much space as the file takes. If
you saved the file attachment to your C: drive, then your c: could be close to
full. If you saved the attachment to any other drive, check that drive for
space.
If you opened the file from your email client, then I bet your email client
saves (without you knowing it) to a special folder on your C: drive. So it
could be your c: drive is getting full.
If you open the workbook from the email again, you can click on:
File|Properties|General Tab
and look at the location to see where that special folder is.
Or you could put this in an empty cell:
=cell("Filename",a1)
don't change anything in that formula