my car tries to turn itself - kinda like Christine-what do ya think

so the other day i was turning around in my parking lot at work and my car started making a whining noise and the steering wheel started to turn by itself--it continues turning until it gets all the way to the right -then i giggle the wheel the other way and it spins all the way the other way. does anyone know what's up--is it just my power steering pump or could it be more? i have a 98. thanks-brody