The Microsoft Surface RT runs a special version of Windows called Windows RT. The Surface RT has an ARM based microprocessor instead of the traditional Intel/x86 compatible processor used with most other Windows based systems. This different processor requires an operating system that is compatible, hence the Windows RT operating system.
One unique aspect to the Windows RT operating system is that malware written for the standard versions of Windows will not run and therfore won't infect a Surface RT device. In addtion, all apps that are capable of being installed on the Surface RT must be downloaded from the Microsoft App store and contain a digital signature that allows them to run. This is another way in which viruses and malware are prevented from running on the Surface RT. This doesn't mean that the Surface RT is immune to viruses, it just means that a virus would have to be crafted specifically for use with Windows RT.