I'm running Excel 2007 on a WinXP (SP2) machine (dual core, 4GB RAM, 8GB free HDD space).

I keep getting a strange error message frequently when I try to save large files (> 2MB) with lots of sheets. Excel seems to save okay (the save progress bar at the bottom finishes), but then a dialog box pops up saying:
Your change could not be saved to filename.xls because of a sharing violation. Trying saving to a different file.
Once I hit Ok on that dialog box, it immediately gives another dialog:
The file you are trying to open, '56C2F10', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now? Y/N
I hit Yes, and it then opens another version of the same file and gives me the Save As... dialog box. Once I enter a different name, it saves that file and closes it, leaving me with the original file that I was trying to save (but could not). I then close that original file and open the new/renamed file and it contains all my changes.

As I mentioned, this happens usually with large files (>2MB), usually ones which have many sheets (>10). It has happened with .xls, .xlsx, and .xlsm formats. I am saving only to my local hard drive, and the file is not shared with/on any other user/server/machine.

It is very annoying, since I have to interrupt my work to close the "corrupted" file and open the newly renamed one, not to mention that I end up with multiple copies of the files.

Any insight as to what's causing this problem, and how to fix it?