Why does the old nature keep a believer from doing the good the new nature wants?
The old Adamic nature is entirely without good, so even when the will to do good is present, the ability to perform it is not. It continually tips the balance against the new nature's desire, and the resolution is deferred to chapter eight.
Les teaches that Paul is describing the conflict from the old Adamic nature's point of view, and from that viewpoint 'there is nothing good.' Paul writes, 'For to will is present with me, but how to perform that which is good I find not.' The old nature keeps tipping the balances, so the believer asks how to do what the new nature wants when the old nature opposes. Paul repeats the struggle in verse 19: 'For the good that I would, I do not: but the evil which I would not, that I do.' Les notes this is where everyone finds themselves, and he warns against the claim that a believer no longer sins. The answer to the question is not given until chapter eight, where the Holy Spirit is mentioned many times.