It would actually be illegal (under EU law) for Microsoft to bundle Excel with their Windows operating system. Microsoft has had to pay massive fines because it has automatically included Internet Explorer with Windows, to the detriment of organisations offering rival browsers.
If you simply need to open an Excel document which someone has sent to you (rather than to create a new spreadsheet of your own), download and install Excel Viewer:
http://www.microsoft.com/en-gb/download/details.aspx?id=10
If you want a free spreadsheet program which is compatible with Excel, download and install OpenOffice:
http://www.openoffice.org/
(It includes Calc, which mirrors Excel in the way that it works).
Chris