Thanks very much for your reply Tweakie.  I had not checked that, but I did, and the folder does exist.  There is even an unistall subfolder there too.  I tried deleting them individually, and together and trying to reinstall. I still get the same error.  I also tried creating a folder on my desktop called Mach3 and installing it there.  I still get the same result.
Although everything else on the computer seems to work fine I'm starting to think that I've got a corrupted OS.  I don't know how else to explain it.  Thanks again, I'm glad to be here on the forum and can't wait to get past this problem and start using my cnc.