Not sure if this is relevant to your situation but this article may assist with evading AV suing metasploit payload. This method offers alot of flexibility to the penetration testers to obfuscate the ASM instructions however they chose. For more information see this blog article here:
If you are using Backtrack 5 try this:
Run Social-Engineer Toolkit
1) Social-Engineering Attacks
4) Create a Payload and Listener
13) ShellCodeExec Alphanum Shellcode
Works to bypass most of the AV.