Security is one of the import concern now days. Malicious software (malware) exploits vulnerabilities in computing system. Malware includes viruses, worms, Trojan horses, spyware that gather information about a computer user and access to a system without permission. It can appear in the form of code, scripts, active content, or other software. Malware programs are divided into 2 classes: first class of malwares needs a host program (viruses, Trojan horses, logic bombs, trapdoors) and second class of malwares are independent programs (worms, zombie). Their prevention is important.