I don't seem to have the proper software to open files with the .docx or .xlsx extensions. I get a "please select a program to open with...." message. Answer:
Office 2007 products (Word, Excel ...) create a new format of it's documents and even give it a new extension : .doc ==> DOCX
.xls ==> XLSX etc...
If you have office 2003 installed on your pc and want to open a document created with Office 2007 you will need to install a small patch for office 2003 that takes care of automatically converting the document so you can open it with the 2003 version.
Installing the following patch will solve this for office 2003: http://office.microsoft.com/en-us/products/HA101686761033.aspx
Details on the patch: Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 file formats