Apparently files saved in Microsoft Word 2007 have a different file extension — .docx — and cannot be opened by earlier versions of Word.
Several weeks ago, one of my clients sent me a file another of her writers had emailed her with; my client wanted to know if I could open it. My poor, confused Office 2000 thought the .docx file was an executable file, so I didn't open it.
I did research it, though, and discovered that the extension was the new format for Word documents. There are several sites online that will convert the file to a regular .doc file — for a fee, of course. Otherwise, you are SOL if you get one of these and don't have Word 2007.
One of my favorite things about Word up until now is that files from different versions are always compatible with whatever version you happen to have. (For me, it's Word 2000.) Imagine having to always have the same version as your clients — when each one of them could use a different version! I think Word's new document format is no doubt going to cause problems for many writers — particularly those who follow the rule of always upgrading to the newest software versions.