Another version of Microsoft Office, another set of file-compatibility hassles. (No, really, thanks a ton, Microsoft.) Office 2007 brought with it the Docx format, which can't easily be opened by anyone who doesn't also have Office 2007.
Option #1: Install the succinctly named Microsoft Office Compatibility Pack for 2007 Office Word, Excel, and PowerPoint File Format. 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 OpenXML Viewer extension, 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 Docx Converter. In short order you'll receive an e-mail with a link to converted version of the file.