cannot install after resetting computer to factory settings

I reset my Windows 7 computer to the factory settings.  After I had successfully done this, I tried to download and re-install my Norton subscription.   At the end of the install I get an "Unrecoverable Error" message, indicating the install is not complete.  The troubleshooting option I am given is to try again. That does not work.  The next solution offered is to run the Norton Removal tool.  I have done this before, and it has solved installing problems.  However, when I download and try to run it, I get "Invalid signature" and the program will not run.  I have tried restarting the computer and retrying the process several times and don't know how to go further.  Thanks for any help.