99 ford taurus door locks

this 99 taurus only has about 60,000 miles on it and the door locks stopped working.When you hit the unlock or lock button the door locks make noise like there trying to work and they even move slightly.But they wont unlock or lock they just try.Does anybody have any ideas of why this is happening?Oh and I had a viper alarm put on it for the keyless entry and it worked for awhile but then this happend and the alarm people said it was the door locks not the alarm.thanks for your time.any info is greatly appreciated