Microsoft Office has so many users that its file formats have become de facto standards. Even alternate operating systems and applications have had to humbly adopt the competitor's format. So I still get annoyed, even two years after its release, that Office 2007 "upgraded" formats and broke this standard. Even now, you'll still run into situations where clients or coworkers don't have the latest version of Office or a patch for an old one. They won't be able to open your documents or import them into an incompatible application. Here's how to set the default file format back to that standard.
In Word 2007, click the circular Office Button in the upper-left, and press Word Options. Click Save, and use the pop-up menu to set the format to Word 97-2003 Document. That switches from the dreaded .docx format to the older, more compatible .doc style.
The process is similar in Excel 2007. Click the Office Button, and press Excel Options. Choose Save. Pick Excel 97-2003 Workbook as the default format in the first pop-up menu.
And in PowerPoint 2007, follow the same path. Click the Office Button, pick PowerPoint Options, click Save, and choose PowerPoint Presentation 97-2003.
Microsoft needed these formats to create some of the newest Office additions, such as copy-protecting files. But most people won't notice the loss of those Office 2007 features, and you'll be able to share work with nearly anyone, regardless of OS or office suite software.