I got this error from a asp.net web page exporting data to an excel file.
From the looks of it, it look mysterious.
But the microsoft kb article is quite useful -> http://support.microsoft.com/kb/316475
I read point #2 from the kb article and set the folder which I am writing to from read-only to write and it solves my problem!
- Make sure that the file that you are trying to open is not saved as