Helping people with computers... one answer at a time.
".docx" is the new extension used by Microsoft Word 2007. There are a couple of approaches if you're exchanging documents with users of older versions.
When I try to attach a document to emails I have discovered that the recipients cannot open them. They're telling me that there's a 'docx' on the end of the subject. I didn't have this problem with my old computer, I now have a gateway XP Pro.
I'm going to guess that with your new computer you also got some new software. In fact, I'm going to further guess one of those new software packages is Microsoft Office 2007.
If that's the case, then what we have here is a classic cross-version compatibility issue. Fortunately there are two ways to solve the problem.
You can solve it, or your recipients can.
".docx" is the new file extension that Microsoft Word 2007 uses when it saves documents in the new default format. So you might save your document as "letter", but what Word 2007 writes to disk is "letter.docx" where previous versions would write "letter.doc". (I assume that ".docx" means .doc "extended", but that's an unimportant assumption.)
As I said there are two possible solutions.
Save in the old format
In Word 2007, rather than just hitting Save, click on the "Office Button" to drop down what we used to think of as the "File" menu, and hover the mouse over Save As:
Note that I've called out the option so save as a Word 97-2003 Document. This will save the file in the format used by these earlier versions of Word, and will do so with the normal ".doc" extension.
If you always want to save in the older compatible format you can click on the Word Options button (always visible at the bottom of the menu displayed with the "Office Button"), and then click on Save at the left of the resulting dialog:
As you can see, you can select the Save files in this format dropdown. The default, as you might expect, is Word Document (*.docx). Change that to Word 97-2003 Document (*.doc) and your documents will be saved in the old compatible way.
Open in the new format
I actually covered this in a previous article: there is a "compatibility pack" that users of older versions of Word (and Excel and PowerPoint) can download and install to enable those versions of the product to read the newer file formats directly.
In your scenario you would do nothing different. You'd keep sending out the new ".docx" documents. Your recipients, however, would all need to download and install the converters to be able to read what you've sent; or upgrade to Office 2007.
Which is the right solution for you depends on your situation. If your recipients expect to have lots of .docx files coming their way or just want to be ready if and when they do, then perhaps asking them to install the converter isn't that big a deal. On the other hand saving in the older formats to begin with minimizes the impact on them - at least for documents that you send.