For some unknown reason, if you are using Microsoft Word 2008 for the Mac, anytime you try to save a file it uses the .docx file extension by default. The problem with saving files in this format is that older version of Word can not open this type of format. I suppose Microsoft thinks that if people save in the .docx format enough, it'll force more people to upgrade to Word 2008. Very annoying since you then need to select the format for every file you save if you just want to use the normal .doc format.

Luckily there is a way to change the default file format used when saving. With Word 2008 running, go to Preferences and select Save. On the panel that you get is an entry called Save Word files as: with a select box next to it. If you select the Word 97-20004 (doc) format, your files will be save in a format that can be read by those not using Word 2008.