With the introduction of Office 2007, Microsoft abandoned the age-old .doc format in favor of the newer .docx (also known as the Office Open XML format). If you're still using Word 2003 or an earlier version of the program (and many people are), you won't be able to open .docx files without a little assistance. Likewise, the venerable .xls Excel spreadsheet format gave way to .xlsx, and PowerPoint .ppt files became .pptx files. Anyone using Office 2007 or Office 2010 is generating these kinds of files, which older versions of the Office applications can't read.
What you need is the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. (Now there's a name that leaves no room for confusion.)
Designed for use with Office 2000, XP, and 2003, the downloadable add-on allows you to open, edit, and save the newer file formats, seamlessly converting the files on the fly and in the background. Just make sure that your copy of Office is equipped with all of the latest updates from Microsoft before you install the Compatibility Pack.
(For more solutions to perplexing PC problems, see "The 21 Greatest PC Mysteries--Solved!")