Sun Microsystems released a plug-in that allows MS Office to work with ODF (or ISO 26300) files. The plug-in supports text-documents, spreadsheets and presentations, corresponding to MS Word, MS Excel, and MS Powerpoint. The plug-in works with MS Office versions 2000, XP and 2003. It can be downloaded from Sun for free.
Why does it not support MS Office 2007? Malte Timmermann gives the answer and many more around the new ODF plug-in for MS Office.