According to Microsoft’s Crispin Cowan, Vista Paved the Way for Secure Windows. He also notes that:
“If you are running as administrator, security is fairly hopeless," he said. Unfettered administrative rights is what allowed malware and viruses to take control of computers.
Vista, featured a total separation between what a user can do on a machine and what an administrator can do, a separation that has always been enforced on Unix distributions. This separation, enforced by UAC, limits the damage that a user can do to a machine.