In "Biblical Perspective" Gordon Snider writes:
In identifying what Paul meant here by old man we must remember that the context emphasizes the total change that takes place at the time of justification. The believer is no longer “in Adam,” but “in Christ.” It seems best to understand our old man as referring to our lives as we lived them before justification. The purpose of the crucifixion of the old man is to break the power of the body of sin. Thus old man and body of sin cannot be synonymous. Some understand the latter as referring to our physical bodies, particularly where the body has become an instrument of sin. Another option is to understand the body of sin as equal to “the body of this death” spoken of in Romans 7:24. The latter would mean that the body of sin is the root of sin from which acts of sin spring. The purpose of both actions is that we should not serve sin. We cannot be recipients of forgiveness without also being recipients of deliverance.
Source: Studies in Romans: Adult Teacher's Insights, page 29.