A zombie is a computer connected to the Internet that has been compromised by a hacker, computer virus or Trojan horse and can be used to perform malicious tasks of one sort or another under remote direction. Botnets of zombie computers are often used to spread e-mail spam and launch denial-of-service attacks. Most owners of zombie computers are unaware that their system is being used in this way. Because the owner tends to be unaware, these computers are metaphorically compared to zombies. These compromised machines send a lot of spam messages on the internet. Such machines result in spamming attacks, DDOS attacks, identity theft which result in different kind of losses to the victim. Spamming botnets is the network of compromised machines involved in spamming. The SPOT, the sequential probability ratio test is used for detecting the compromised machines. SPRT is used since the error rate produced is infinitesimally small and the number of observations required to deciding whether a machine is compromised or not is also small. It helps in observing the outgoing messages from a machine in a network. Out of a large number of machines in a network only a few of them are not compromised. For an instance, out of 440 internal IP addresses SPOT identifies 132 of them as being compromised. This system has been developed for system administrators for monitoring the machines in a network.