Unable to open zip files with Archive Utility or StuffIt

Hello,

I am running Mac OS X 10.7.3. I have been attempting to download and install the Eclipse Java IDE from both www.eclipse.org and from a Stanford course website. In the first case the file is offered as a tar.gz and in the second case it is a .zip. When I attempt to open either file using Archive Utility or StuffIt Expander I get a never ending pinwheel. I am forced to restart to get Finder working porperly again and quiet the fan down. My wife's computer, also running Mac OS X 10.7.3, can extract the files without a problem.