Quantum computing takes a different approach than normal computing does. As far as my understanding goes, they are more likely to specialize in a certain area than a classical computer is. Take for example a problem that can have a lot of variables and possibly trillions of combination answers. A classical supercomputer might fail at this task because the brute force computing it does would require memory that it simply cannot possess. A quantum computer might instead create a model where patterns may emerge, with the best combination of patterns being the answer. This is however a very rudimentary way of explaining the differences.

As regards to the possibility of there being Quantum PCs, the chances seem very slim. For one, Quantum computers require their processors to be super-cooled, we are talking at the very least a few degrees above absolute zero. They may become more common, but Quantum PCs don't seem achievable, at least for now.