I am doing a fresh install each time. I have nothing PTC related installed on my computer.
Drive C: has about 15GB available (required is 3.4 max)
Drive D: has hundreds of GB available.
For clarity, the installer never makes it to the installation step. It gets to the point where I can select where to install it, and then exhibits the behavior described in my original post. It will not let me proceed to installation under any circumstances except installing to drive C: (and then, only if I don't change the drive first).