One key thing to be aware of is the new format that Office 2007 saves documents in, ‘X’. Word will save documents as a ‘.docx’, Excel will save spreadsheets as ‘.xlsx’, etc. This new format is XML based and is not readable by older versions of Office unless you have installed Microsoft’s free compatibility tool on the older versions of Office which can be downloaded at:
http://www.microsoft.com/downloads/details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-C6BB74CD1466&displaylang=en . Keep in mind that you can always save Office 2007 documents in the older, fully compatible formats (.doc, .xls, etc.).

If you are using Excel 2000, the 2007 formats are not added to the “save as” dialogs within Excel. Also, you cannot use “File → Open…” to open an Excel 2007 document in Excel 2000. You must double-click the document from windows explorer. Other requirements are detailed here:
http://support.microsoft.com/kb/924074