abs whirring, can't stop

I have a 99 sunfire. My abs come on when not needed, and most times, seem to knock out all braking power. I hear the usual thump, thump sound, but also a whirring/clicking sound. Then the brake pedal becomes extremely difficult to push down and I have to sometimes use two feet. I just keep on gliding until finally they kick in and stop the car. I took the car to a dealer and he said the abs were fine. He just took it for a drive to determine this. Is there any other way of testing them besides just taking the car for a drive? I am so afraid because another Canadian winter is coming up and I have to try to avoid braking. I contacted Pontiac and mentioned that I might have them disabled and the rep said if I did, and I had an accident, the insurance wouldn't cover it. Is that right? Please-any info would be so much appreciated.
Melody