Hi cat welcome. Those are good questions.
First of all let me start by saying that nothing is off limits when it comes to malware and viruses. If there are enough people using a device some hacker will figure out how to target it. That includes, smart phones, tablets and PCs (including Apple products).
Having said that Windows has come a long way in protecting itself against these kinds of threats. Microsoft got so serious about it they created their own "anti-virus" suite Windows Defender followed by Windows Security Essentials. Both of these are very good at protecting Windows. Of course see my point above since anything can be beaten.
Since Windows RT is a new OS it should be relatively free of malware and viruses. It is based on traditional Windows though so it may not be too difficult for hackers to port their software over at some point in the future.
The real threat to the security of your device comes from the user. The best way to avoid malware and viruses is to (1) only download apps and programs from authorized retailers (like the Microsoft App Store, Google Play Store and iTunes). (2) Do not visit sites you are not familiar with. (3) Do not click on links in emails that you are unsure about. (4) Do not open attachments in emails you are unsure about. The majority of people who have problems with malicious software on their devices did not follow these 4 simple precautions.
So in some sense RT might be slightly more immune to malware and viruses but I wouldn't count on that being a feature. Furthermore, if you neglect to take the proper precautions to avoid malware and viruses in the first place it doesn't really matter what device you are using.