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.