Aren't modern Windows capable of installing everything as root and keep the user account from infecting anything that is on the system level? That effectively

Aren't modern Windows capable of installing everything as root and keep the user account from infecting anything that is on the system level? That effectively solves the virus problem since the worst that can happen is that something unwanted runs as the user privileges or deletes/infects files in home directory. The machine itself stays clean and you can avoid full reinstalls. If the user gets a virus then all you need is restore his home directory from a clean backup. And if you want, possibly run some antivirus on anything that gets backed up, to try to make yourself feel good about backups being clean.