Jmol may be one of the easiest viewers available. All you need is a web browser and Java installed.
From there, you just need to find the Protein Data Bank code for whatever it is you want to view, and Jmol autoloads the model for you. What could be easier?
A barrel-shaped protein that encloses itself...
If that isn't enough, try looking at it with the Jena3D / JenaLab version of Jmol, even more pretty!