You could use 7zip's command line version without installing it.
http://www.7-zip.org/download.html
Also, if your excel files are outputed as '.xlsx' they are already compressed. This is also true for any other office type that ends in an 'x'. Try changing the extension to '.zip' and open them.
Edit: Depending on how much you're doing you might want to consider buying the Cozyroc package which includes a zip task as well as many other useful ones. Not too costly either.