Excel Web Sharing

I have a client that is in desperate need to share Excel 2000 and beyond (XP) documents through the web. He wants to be able to post an Excel Document on a web site that is edited/updated by one or more privledged individuals, and be able to viewand download these updates into the local copy of Excel that is running on his workstation.

Is this only possible with the newest version of Excel, or are there add-ons or utilities that can make this possible? I am already aware of how to post a static version of an Excel Workbook to the web, and want to take it just a step further.

Thanks for the help!

Paul M. Chavez