How Can I Convert DOCX Files To DOC?

I just received a Word document from a client and I can't open it. When I looked at the file, I noticed that the file extension is DOCX and not DOC. I believe this is the new format for Word documents for Microsoft Word 2007 and beyond. And since I only have a Mac and a MS Word XP in my PC, I cannot open DOCX files. Good thing that I was able to find this article that explains in a simple way how I can convert DOCX to DOC and also how to open DOCX files in Macs. If you have the same problem, then just read this great post. Cheers!