I read that Sharepoint has a feature called Excel Services which bascially lets you use excel via web browser even if the computer does not have office installed.
http://office.microsoft.com/en-us/sharepointserver/ha101054761033.aspx

How come they don't have Word and Powerpoint services? I think I remember you can work on a word doc or powerpoint deck in the browser in Sharepoint even if you don't have office installed on the computer.