Nobody should be allowed to install anything without admin privileges.
You are wrong. If you are using a business computer, you shouldn't able to install anything without admin privileges but for home users, it’s too complicated to switch from admin account to regular account. That why most home users (including me) keep logging and stay log into their admin account. And when they catch malware, the malware can do anything because they are administrator. That why Microsoft is trying to change in Longhorn with this paper.