defregmenting problem

Hi, Im not sure if you understand what I mean,but Im trying to defregment,it wont let me do this in drive C as it went down to only 7%,I know when I usaully defregment the red colored will go, but this time it wont go, so that means there are files somewhere which need being deleted only I dont know where they are, can anyone give me a hint pls? tks