You can get a old upgrade license for 2007 on Amazon, better bet than ebay. Ebay has way too many fake or pirated licenses. http://www.amazon.com/Microsoft-Office-Standard-UPGRADE-Version/dp/B000HCVR3K
As stated above, it's not free.
OpenOffice is great to use, but keep in mind that some formatting may not look exactly right and the spreadsheet in OpenOffice is not as good.