Evading Anti Virus

Read these links

• https://community.rapid7.com/community/metasploit/blog/2014/03/26/new-metasploit-49-helps-evade-anti-virus-solutions-test-network-segmentation-and-increase-productivity-for-penetration-testers
• http://www.scriptjunkie.us/2011/04/why-encoding-does-not-matter-and-how-metasploit-generates-exes/
• http://schierlm.users.sourceforge.net/avevasion.html
• http://www.pentestgeek.com/2012/01/25/using-metasm-to-avoid-antivirus-detection-ghost-writing-asm/

There are approximately 14 million other resources out there on the why's and wherefores of evading antivirus, but the about articles should get you started.