In Microsoft Word 2007 and 2010, you can save an individual document in Word 2003 version by clicking the Office button in 2007 or the File Menu in 2010, then Save As, and changing the Save As Type to Word 97 – 2003 as shown below.  If you’ve used Word 2007/2010 features that aren’t available in Word 2003 (such as smart art) you may see a message that says you won’t be able to edit such objects in the earlier version.

If you frequently email or share files with Word 2003 users, you may want this to be the default file format for all your word documents.  In that case, click the Office Button in 2007 or the File Menu in 2010, choose Options, then click Save on the left and change the file type there – see screen shot below: