Better default installation path

Installing directly to `C:` by default is a little obnoxious, especially with the vague warning about permission issues (which makes no sense, since `C:` typically has the strictest permissions.)

It gives no idea what the installer is actually going to install, or what permissions it requires, making it hard to override it with a better choice.

Binaries should normally be installed to the "program files" directory, and user-specific configuration to the user's AppData folder.