Alright, Cheetah is back in the running! Thanks for the idea about permissions. It still wouldn't install, no matter where I moved the file, or if I ran as Admin, but I got the thought to download an older version, like 7.6.3, and ran the installer. Sure enough, it came up with a repair option, and then took a full 2 mins (on a fast rig with SSD) to perform "Setting permissions for BOINC folder". So something was off there, and whatever the old installer did, fixed it.
Then, I was able to install the new 7.8.3 version right on top of it and works like a champ.