>>42665767really the only thing you can fuck up installing is git and python, WebUI installs everything it needs into a little Python "virtual environment" the first time you launch it. "Install for all users" is greyed out and "hard selected" for me because I already have it installed for all users, and I think it defaults to doing that when you launch the installer as an administrator. As long as you see it installing it to "Program Files" you're set. If it's installing to your user folder, it will still work, but it'll be more annoying for any other shit you want to do with Python in the future...
>>42666156stable-diffusion itself will end up taking like 60GB when you have it up and running, so when you do "git clone
https://github.com/AUTOMATIC1111/stable-diffusion-webui/" - you can do that onto the drive you want it on. Python and Git installing to C:\ is fine, you don't want them on another drive. Especially not Git, it's fucking retarded if you install it to a different drive letter and then remove it.