Ray - I would go to http://download.openoffice.org/ and download OpenOffice. It has an spreadsheet program like Excel called "Calc" along with other MS Office like programs. If you can't afford or don't want to spend the money on MS OFfice, then OpenOffice from Sun is a great alternative.
Then you can calculate negative logs to your heart's content!
Also, if you're a Windows person, search around for Microsoft's scientific calculator for windows - under accessories under programs under the start button. It has a scientific mode. I'm sure Mac/linux has something you can download as well.