Collections

About 5 years ago i bought a PC from Dell. A month after i recived my CPU i noticed my payment didnt get taken out of my account, so i call Dell, they assure me its okay it will come out as a double payment next month. Next month comes around still no payment taken out, call them again and they tell me i was sent to collections for non-payment... i tell them ive been trying to pay, but they say nothing they can do. So collections calls, i explain what happened and paid the full ammount. Well now as of yesturday i get a credit alert that Dell has sent me to collections.... what a joke, all because of their mess up. Whats the best route to take to fix this?