Even a Mac is only a computer!! Even a Mac can fail and yes, even OS X has flaws, including security flaws.
It certainly is more difficult to make a virus for any Unix-based system, nevertheless it IS possible, and just because of the limited user-base nobody seems to be interested to create such thing.
The registry definitely is Windows' weakest point, but more because the way it's handled than the idea itself.
Don't get me wrong, I like and always prefer a Mac over a PC, and OS X is really my favorite OS at the moment, nevertheless, even Windows has it's good points, just as OS X has it's bad points.
I use both, OS X at home and Windows at work.
Simply stop that nonsense bashing of other platforms. It makes absolutely no sense, and if Windows really were so extremely bad, how come that today's world, which is heavily relying on PCs running Windows, didn't collapse yet? So, it can't be that bad, can it?