A quantum computer is not "infinitely parallel" and though it's still an open problem, it's considered unlikely that that quantum computers can solve NP-complete problems in polynomial time.
Explain
If you use the metaphor that superposition is like computing many things in paralell, the problem comes in that when you measure. The superposition collapses to a single answer at random (with probability related to the amplitude of each possibility) which will usually not be the answer you're interested in.
For some problems, people have found ways to extract useful information via classical measurements, e.g. Shor's algorithm (in theory breaking RSA/DSA/ECDSA/DH/ECDH style public-key algorithms [1]). However in the general case this does not work (so AES and hash algorithms are safe for now).
[1] https://en.wikipedia.org/wiki/Wave_function_collapse
[2] https://en.wikipedia.org/wiki/Shor%27s_algorithm#Quantum_par...