checkbox cannot be checked by users

Hi ,
I have created a form where the checkbox has to be checked by the users .But the form automatically gets the value as checked or unchecked depending on if i have typed -1 or 0 in the checkbox properties ,the check boxes are created with in the options group.There are two check boxes ,one should be checked if yes the other is No if unchecked .Can someone please suggest a solution
regards
Sumona