| Congratulations - you've hit on the business model which made Bill Gates the richest man in the world.*
Microsoft make a habit out of changing their file formats in every new version of their software, so that their upgrade revenues grow virally - in order to open the Office 2007 file, you have to have a copy of Office 2007. The Office 200X version you have from a year or two back will not open the file.
[*] Yes, alright, he's been doing good stuff with the money recently, but that doesn't change the dodgy way it was obtained. | 
