Keeping your computer clean and secure is extremely important, but there are a lot of people who will try to benefit from it. Like the proverbial bad apple, this bad system-cleaning app or software can hurt your system by pretending to do the job, but actually only simulate it leaving your computer vulnerable to viruses and malware.
Although it comes in many forms, these fake programs (called rogueware) are referred to antivirus or system cleaning software which pretends to be a real antivirus application. It runs a simulation and tells you that your system is heavily infected with Trojans, viruses, and worms and offers to buy its services to remove the problem. Sometimes they will tell you that your hard drive is going to die and a premium version of a product can recover your files.