If you use Microsoft Office products (Word, Excel or PowerPoint) version 2000, 2002 (aka XP) or 2003 and would like to be compatible with the new file formats for the 2007 versions of those applications, Microsoft released a 'free' new compatibility pack that will enable you to view, edit and save your files to those new formats. This is pretty nice, as you will no longer have to tell co-workers/co-students/co-anybody using Office 2007 to save stuff in the old formats so you can use them.
Before you run off to install it, there are some caveats. You MUST upgrade your version of Office (or standalone Word, Excel or PowerPoint) to the very latest service packs available.
If you need direct links to the various required Service Packs:
Office 2000: SP3 - http://www.microsoft.com/downloads/details.aspx?FamilyID=5c011c70-47d0-4306-9fa4-8e92d36332fe&DisplayLang=en
Office 2002 (XP): SP3 - http://www.microsoft.com/downloads/details.aspx?FamilyID=85af7bfd-6f69-4289-8bd1-eb966bcdfb5e&DisplayLang=en
Office 2003: SP2 - http://www.microsoft.com/downloads/details.aspx?FamilyID=57e27a97-2db6-4654-9db6-ec7d5b4dd867&DisplayLang=en
Those Service Packs work for all full versions of Office, as well as the standalone versions of the three main applications included.
In turn you must also get the most recent critical fixes published as of April 10, 2007 using the Office or Microsoft Update site.
Once you have finished all the updates needed, go and install the Compatibility Pack from http://www.microsoft.com/downloads/details.aspx?familyid=941b3470-3ae9-4aee-8f43-c6bb74cd1466&displaylang=en&tm