When an Office 2007 document is placed in a SharePoint 2003 document library, downloads of the document cause the extension to be changed from the new extensions to .zip.
This is not a SharePoint error!
This action occurs because the extension is not recognized as a registered mime type. Frequently, the first tendency is to register the new mime type in the server and client registries. The error is occuring, however, in IIS 6.0.
The solution is to open IIS Manager, open the properties of the web server and register the mime type for the extensions. This can be accomplished for individual servers but will be easier to do globally in the web server properties. An IIS reset was not necessary in my experience.