After upgrading to Microsoft® Office 2007 is the default for all documents is the .???x format for example new Word documents now end in .docx instead of .doc.
Although the new file type offers many improvements over the old one, it can be troublesome to use if you work with many people that have older versions of Office. There are workarounds available from Microsoft, but it's usually easier just to send them something you know they can open.
One way to solve the problem is to simply change the default file format to the previous Office formats. In the case of Word 2007 that’s to DOC.
To do this go into Word 2007 and click the Office Button | Word Options | Save | Save files in this format | Set this to "Word 97-2003 Document (*.doc)" and click OK.
For Excel and it’s the same principle.