If you use Office 2007 and want to send documents to someone using Office 2003 so that they can open them without installing the compatability pack onto their PC, you can tell your PC to save in the 2003 format:

One off, or file-by-file:

These steps show Word 2007, but the process is the same in Excel 2007 too. Click the "Office button" at the top left of your screen and select Save As. You'll see Word 97-2003 as an option - select that and then type the name you'd like to use for your file:

 
Change the default so you don't have to choose 2003 each time:

If you find yourself doing that all of the time, you can set the default filetype to the Word 2003 format instead: click the "Office button" and then select Word Options at the bottom of the menu:

In the Word Options window that opens, click the Save option in the left column and then pull down the menu titled Save files in this format and select " Word 97-2003 document (.doc)" and then click OK:

Once you've done that, Word 2007 will default to saving in the traditional Word 2003 file format, and everyone will be able to open documents that you send them without first having to download an update for their PC.