Office 2007 has a little different design than Office 2003 and some of us liked the old version and didn't upgrade. Because Word 2007 saves files with the default extension .docx which is not compatible with Word 2003, there were a lot of misunderstandings regarding extensions.
But Microsoft came with a solution for this problem called Microsoft Compatibility Pack.
To install it follow this steps:
1. Go here and download the pack.
2. After you saved it on your computer open it and after you read the license click Continue.
3. Once the process is done , you can enjoy opening .docx files on Word 2003.