The easiest way to enable support for the new Office 2007 file formats in Office XP is to install the Migration pack provided by Microsoft free of cost. With the introduction of the XML standard in file format, the new files with extensions ending in x (docx, xslx, pptx) will not be compatible with the older Microsoft Office.
With Migration pack installed you can either save in all the New office supported formats as well as open all the new format files. Here is a screen shot of the Save As dialogue in Office XP with 2007 compatibility pack installed.
Main advantages f the new file format are that it is a standard XML format and also the compression ratio is better for the XML formats. You can open all the 2007 Office files using any archiving tool.
The Office 1007 compatibility pack is free of cost and can be downloaded form Microsoft Download center. You must install all High Priority updates from Microsoft update center before installing this pack. Here is the Office 2007 compatibility pack download page at Microsoft Download Center. The file is a 27.5Mb Windows Executable.