Child porn

PLEASE HELP! Not only have I discovered child porn on my laptop that was loaned out to someone else, now the police are sounding as if I might be in trouble for having it. I contacted them to find out what to do, if they could remove the files from my computer, and they make it sound as if I did something wrong. It's my computer, but not my files, and I went to them for help. Do I need legal help? Can I hold the one who put this on my computer legally responsible for it and make him pay for having it professionally cleaned? I don't know what to do now, but I've already given my name and address to the police and they are making me very nervous. I'm supposed to turn it over to them tomorrow and am afraid I'll end up in jail for innocently finding something and turning to the police for help.