If you have a bad credit history, anyone who's unconditionally guaranteeing to erase it for you and get you new credit is probably promoting a scam. If they're promising to remove bankruptcy and liens from your credit file, get you a bank credit card without a credit check, or make other claims that seem too good to be true, they probably are.
Federal officials report that credit-repair scams cost consumers millions of dollars each year.
On the other hand, there are legitimate, professional credit counselors. Some charge modest fees, and many offer a free credit analysis. Your local Better Business Bureau also can put you in touch with legitimate debt counselors, as well as letting you know whether or not a certain service you're considering may be a come-on.