Depends on the spell.
Different spell have different components. Some have somatic components, some have material components. Both of these require using your hands.
Still spell only removes the somatic component requirement of spells. You would require eschew materials to remove material component requires (but only those that do not have a specific GP cost).
However, why do you need to knock an arrow at the same time as casting a spell?
You can hold the bow in one hand (removing you hand as a free action), begin casting the spell with one free hand (which is capable of providing both material and somatic components), finish casting the spell, and then as a free action place your hand back on the bow. It's also worth noting that nocking an arrow is a non-action in game terms.
Not an Action: Some activities are so minor that they are not even considered free actions. They literally don't take any time at all to do and are considered an inherent part of doing something else, such as nocking an arrow as part of an attack with a bow.