Change your Default Office Version

To Change your file association or default Office version:
(example is setting default Word version (.DOC extension)

Go to your desktop and open your My Documents folder
Click Tools, Folder Options, File Types
Scroll down through the extensions to DOC. File Type should say Microsoft Office Word 97-2003 Document
Select it and Click the Change Button.
Find the square icon for Word 2003 and click it. 
Click Ok and then Close.

File extensions for other Office software are as follows:

Word 2003 - DOC
Word 2007 - DOCX

Excel 2003 - XLS
Excel 2007 - XLSX

Powerpoint 2003 - PPT
Powerpoint 2007 - PPTX

Publisher 2003 - PUB
Publisher 2007 - PUB

Access 2003 - MDB
Access 2007 - ACCDB (IMPORTANT: For databases, once you convert to Access 2007, you may not be able to convert back to 2003)