Open xlsx in Office 2003

I got a VSTO addin which attaches to the Open event. I have installed the Office 2007 Compatibility Pack for office 2003, and then tried to open an xlsx file. In the open even handler, I am getting a workbook passed in but with unexpected parameters. The filename is of the form "CVT5CB.tmp" and the location is the Temp folder. I have also tried with a Shared Add-in and it has the same issue. So I dont know whether this is a problem with VSTO or the addin. Can anyone tell me whether this is a bug in Office Compatibility Pack? Please let me know how to solve this issue? Thanks.

It is not at all possible. You have to upgrade Office to 2007 to run the most latest file format. Or else it just a waste of time. It is necessary that you must upgrade. Any Office 2007 format does not work on Office 2003. The most you can do is simply download a Office 2007 file viewer that will allow you to see the files only. There is no way to modify it.