Well there's whole books written on that [what Paul is talking about in Romans] and I wouldn't claim to be a authority -I do know that Paul mentions several times that it impossible to live by the law so now its time to look up verses.
Romans 2:17-29 (emphasis on 29) tells about the mosaic law and how it is an inner law, a circumsician of the heart.
So the mosaic law is written on the hearts of all men although not often listened to or obeyed, it may be what we could call a concience... CS Lewis implies this in his book Mere Christianity, a very good read.
5:20 The law was added so that the trespass might increase. But where sin increased, grace increased all the more.
6:15What then? Shall we sin because we are not under law but under grace? By no means!
7:1 Do you not know, brothers—for I am speaking to men who know the law—that the law has authority over a man only as long as he lives?
7: 7 What shall we say, then? Is the law sin? Certainly not! Indeed I would not have known what sin was except through the law. For I would not have known what coveting really was if the law had not said, "Do not covet."
7:10 I found that the very commandment [the law] that was intended to bring life actually brought death.
7:14 We know that the law is spiritual; but I am unspiritual, sold as a slave to sin.
[b] 7:21 So I find this law at work: When I want to do good, evil is right there with me. 22For in my inner being I delight in God's law; 23but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. 24What a wretched man I am! Who will rescue me from this body of death? 25Thanks be to God—through Jesus Christ our Lord! [u] So then, I myself in my mind am a slave to God's law, but in the sinful nature a slave to the law of sin.[/u][/b]
Note: there are two laws at work here according to Paul.
And finally!!!
8:1 Therefore, there is now no condemnation for those who are in Christ Jesus, 2 [b][u]because through Christ Jesus the law of the Spirit of life set me free from the law of sin and death.[/u][/b]
In Romans 2 Paul is clearly talking about the Mosaic Law which is the law of sin and death and in Hebrews 12 the writer is talking about the Law of the Spirit!
Zac Poonen talks about this in depth in several messages:
Basic Teachings 21 : Law and Grace
http://www.cfcindia.net/tbsaudio/25_Basic_Christian_Teachings/21_Law_And_Grace.mp3
Topical Studies: A Better Covenant 1-6
http://www.cfcindia.com/web/mainpages/topical_bible_study.php#betterCovenant