1. A method of using a computer to determine a reputation of an object in a reputation system, comprising: receiving reports from clients in the reputation system, the reports identifying an object detected at the clients; determining a prevalence of the object on the clients in the reputation system based on the reports received from the clients; determining information about the clients from the reports received from the clients, wherein the determined information about the clients includes an age of a client in the reputation system and the age of the client is determined responsive to an elapsed time that the client has been active in the reputation system; generating confidence metrics for the clients responsive to the determined information about the clients, the confidence metrics indicating amounts of confidence in the veracity of the reports received from the clients, wherein a confidence metric for the client is based at least in part on the age of the client, higher confidence metrics for the clients indicate that information in reports received from the clients is more likely to be true, and an older client receives a higher confidence metric; calculating a reputation score of the object responsive at least in part to the reports received from the clients, the prevalence of the object, and the confidence metrics for the clients, wherein a higher prevalence causes the object to receive a higher reputation score indicating that the object is unlikely to contain malicious software; and storing the reputation score of the object.