Yes, avast! is detecting the Combo-Fix.sys file within ComboFix, at the moment, the best that you can do is report it as a false positive. The more people that do, the quicker they see the problem and can correct it.
I have reported it also.
Send the file in a password protected archive to avast! with the subject 'False Positive' to virus(at)avast.com
Include the info and password in the body of the email.
Scott