Animal Care Needed

My cat is sassy and loveable. She seems pretty smart to me. Which is why it's aggravating that she has a biting problem I haven't been able to make her stop and she's messing in the house even though she has Two perfectly workable litter boxes. It seems that she messes in the house when she's mad, and twice (not in a row) she's messed Right on top of the vent. If we can't get her to behave, my family has threatened to find her a new home, so I need an efficient and hopefully fast solution.