Amazon Affiliate Link: 2007 Microsoft Office System Plain & Simple
The following link is to the OpenOffice.org documentation website, however the information and links provided are relevant to a number of ODF-based Office Productivity Application Software (apps), including:
If you have files with a docx, xlsx, or pptx extension and have NOT upgraded your software lately, then you should consider firstly upgrading your software to the latest version, as they are all able to open these files.
It is important to note that they cannot save in the OOXML formats – only open them and allow you to save in the doc, xls or ppt formats. See this note on file compatibility between OpenOffice.org and MS Office.
If this is not an option for you, then proceed to the link below for at least 10 other options that you can try. One that is not listed at the time of writing this, however, is the use of Google Docs to open the file, then download in a format that you prefer.