Vista - Excel file handling under Vista Ultimate

I'm running Office 2007 Enterprise under Vista Ultimate; when working on a
rather large Excel spreadsheet I've noticed that sometimes when I save it,
Vista complains that it managed to save, but could reopen due to access
share violation (the file is still open, but becomes read-only). I'm advised
to close and reopen it in order to enable editing; even than it happens that
the file is re-open in read-only mode (only this time I'm notified after a
while about releasing it for writing).

Now, this is very weird, as I am the only user on the system, and a one with
administrative privileges; what is the reason of this behavior?

The same file in Excel 2007 under XP pro is handled normally.

Cheers
Piotr