In Microsoft Word 2007, the default extension of a document is docx.  If you have a pre 2007 Microsoft Office installed on your PC, you will not be able to read it unless it is converted into a doc format.

Zamzar.com is one of the site which I found recently on the internet that does this action for you free of cost.  You will have to upload your docx file to zamzar.com and they will mail you the converted doc file to your email address.

Apart from the doc format, zamzar.com gives you a wide range of extension to be converted to

Give it a try at www.zamzar.com
Alternatively you can also visit  http://docx-converter.com