I've just encountered this error when trying to update to the 3.5.2rc1 release (64-bit Python, Windows 10). I'd already had the 3.5.1 release installed, so I suppose it could have been an issue with the older installer trying to uninstall the old version.

I believe the issue may be related to installing the `py.exe` launcher program. I have an alpha release of Python 3.6.0 installed, and so the 3.5.2rc1 installer gave me a popup saying that the launcher would not be updated since I already had a newer version. I assumed that was unrelated to the issue though because the installation process kept going for a couple minutes after seeing that message. However, trying the install again with custom settings and unselecting the launcher program made it work. I'll attach log the main log file for both the failed and successful runs of the installer, and the launcher_AllUsers file from the failed run.

Oh, I just realized the successful run was with the web installer while the failed runs were the full download version. I have no idea if that would have made a difference or not.