Windows RT tablets are not full Windows tablets. They are based upon the ARM architecture, which is found in mobile devices like mobile phones.
Normal Windows as you know it is x86, which is found in laptops and desktops.
Both are inherently incompatible.
So you can only run Store apps on the RT, much like how you can only run iOS apps on the iPad (and can't run OSX apps on it)
On the plus side, since normal apps cant run, virii can't either. (so Avast or AV is not needed)