do you mean that you can't uncompress the windows archive that's on the site (http://mupen64.emulation64.com) ? if that's the case, i can tell that this archive doesn't have any problem as many people have downloaded it and installed it without a problem. I'd suggest to delete it, to delete all your internet browser cache and to download it again.
Thank you that did it! I don't know why the browser cache had anything to do with the software install since I was not running the install from a temporary folder. First I saved it to disk then ran it. Hmm oh well. Thanks again!