Fraud from 4 years ago.

Back in 2005 there was $20,000 worth of fraud put on two of my Discover Cards. I talked with them and told them it was fraud and they said they would investigate. I kept getting the monthly statements but I refused to pay the minimum which was $300 a month on something that wasnt mine. 3 months went by when I got a call from Zwicker and Associates debt collection company. They said they got the debt from Discover Card and wanted there money. I told them It was fraud, they said they would call Discover back and get back to me. I heard nothing back from either Discover or Zwicker. The monthly statements stopped comming as well. Now I know I should of followed up on this and I didnt. Now in 2009 when I go to buy a new car I see both of these Discover balances on my credit report showing negative and all accounts charged off. I call Discover and they say they have nothing on file about fraud and I owe this money still. Since its been over 4 years and I have no paperwork to prove anything what do I do? I am sick just thinking I may owe money I did not spend. I live in Wisconsin and I dont know if there are laws for this.