Have you ever been at work or at home and received a Word document from a co-worker, friend, or family member and observed that the extension (.xls) is different from what you have (.xlsx). The reason for this mismatch is the different versions of Office being used. The newer and latest version (for the time being until Office 2010 is released later this year) has the .xlsx extension. Don't worry if you are in this boat, all is not lost and you do not have to expect imminent doom and file corruption.  Word 2007 will automatically open the file in Compatibility Mode but not all features will be available to you. Thus when you are done editing and close the file, it will be saved in its original format and corresponding extension.

Here are some steps to follow to avoid the continual file mismatch:

1. Click the Office button -> Click Convert
2. In the Microsoft Office Word dialog box -> Click OK -> Click Save

Caveat: You may want to make a backup copy of the file before you upgrade it. Also, to see when you are working in Compatibility Mode, look at the top of the screen ['Compatibility Mode' will appear in brackets to the right of the file name]