With every new version of Office, Micrsoft keeps doing things to 'improve' the product. Mostly these are in the form of new features like Office 2007's ribbon, which is supposed to make it easier to use, but just gets in the way.
One of the things that Microsoft used to change a lot was the default file format. For a few years, every new version of Microsoft Word or Excel had a new file format which was only semi-compatible with the previous one. Microsoft seemed to use file format compatibility as a way to force-march people into upgrading to the newest version of Office.
Things had stabilized somewhat after the introduction of Office 2000 through Office 2003. However, this TechRepublic Word TechMail Tip from April 28, 2000 shows how long the problem has been around and how you could get around it:WORD TIP TECHMAIL
Today's Word Tip
SPECIFYING FILE COMPATIBILITY
As different versions of Word have been released over the years, there
are noticeable differences in the way text layout and formatting
appear. Sometimes it's necessary to make changes to an existing
document so that it emulates the formatting of a previous version of
Word. Having this ability makes viewing these types of documents
easier. The changes you make aren't permanent, so you can change things
back to the old display if you need to.
1. Open the document.
2. Go to Tools | Options | Compatibility.
3. Click on the Font Substitution button to substitute non-supported
fonts with supported ones, and click OK.
4. Choose the version of Word you want to emulate from the Recommended
Options For drop-down list.
5. Enable the appropriate check boxes in the Options list to select any
additional desired display aspects.
6. Click OK.
Today we get to deal with compatibility problems between Office 2007 and previous versions. Office 2007 is supposed to be able to open standard 2003 and previous files, but you still have font problems occasionally. If you're using an older version of Office, you can download the Compatibility Pack which is supposed to make OpenXML documents work in Office 2003, but it's kind of a hit and miss affair.