Microsoft Word 2007 has a new file format (docx) that is not compatible with previous version of Microsoft Word (which can only open doc files). You can save Microsoft Word 2007 files in the doc format. But if you are trying to open a docx file and you don't have Word 2007, you will not be able to open the file.

Zamzar (http://www.zamzar.com/) is offering a free on-line conversion of docx files to the doc format. It's a free service on their site. Just upload your files (you can upload multiple files, up to 100 MB per month on the free plan), give them your email address. They then convert the files and email you the results.