Option #1: Install the succinctly named . It enables older versions of Office to open Office 2007 documents, and it does all the necessary conversion behind the scenes: no input required from you. (Just make sure you've installed the latest service pack for your version of Office, otherwise the Compatibility Pack won't work.)
Option #2: If you're a Firefox user, install the , which lets you open Word 2007 files right in your browser with all the formatting and layout preserved. You can't edit the document, but at least you'll be able to view it.
Option #3: Upload the file to . In short order you'll receive an e-mail with a link to converted version of the file.