Here are several interpretations of Romans 7 and particularly of statements like "the good I want to do I don't do; but the bad I do not want to do, that I do." Some people mix and match these views a bit.
1. James D. G. Dunn: "In this 'I' Paul includes himself as a believer ..., not just in his pre-Christian days... The split Paul is about to expound is one between the epochs of Adam and Christ: the 'I' is split and the law is split ... because each belongs to two epochs at the same time in this period of overlap between the epoch of Adam and the epoch of Christ, between the era of the flesh and the era of the Spirit" (Word, 388).
This somewhat traditional interpretation sees the struggle of Romans 7 as the current struggle of all believers, who live in two worlds at the same time. The one is the world of the Spirit that we serve with our minds. The other is the world of the flesh in which we struggle with sin. This view has fallen on hard times.
2. Douglas Moo: "ego [Greek for "I"] denotes Paul himself but ... the events depicted in these verses were not all experiences personally and consciously by the Apostle... a combination of the autobiographical view with the view that identifies ego with Israel." (NICNT, 431)
"vv. 14-25 describe the situation of an unregenerate person. Specifically, I think that Paul is looking back, from his Christian understanding, to the situation of himself, and other Jews like him, living under the law of Moses." (447-48).
This is a respectable position I think. Moo believes that these verses refer to Paul's past experiences as a Jew and, indeed, to the common experiences of Israel as it has tried to keep the Mosaic Law.
3. Krister Stendahl: "We look in vain for a statement in which Paul would speak about himself as an actual sinner" ("Paul and the Introspective Conscience," Paul Among Jews and Gentiles, 91). "Paul here is involved in an argument about the Law; he is not primarily concerned about man's or his own cloven ego or predicament" (92). "'Now if I do what I do not want, then it is not I who do it, but the sin which dwells in me.' The argument is one of acquittal of the ego, not one of utter contrition. Such a line of thought would be impossible if Paul's intention were to describe man's (sic) predicament."
Stendahl, following the classic study of W. G. Kümmel (Römer 7 und die Bekehrung des Paulus), sees the "I" and present tense of Romans 7 as a rhetorical device to make the situation of the person under the Jewish Law vivid. Such a person may want to keep the Law, but because of the power of sin over their flesh they are unable to keep the Law. But Paul is not necessarily giving some deeply autobiographical snippet of his past (in fact what he says throughout his letters leads us in a different direction, one in which he was very confident of himself in the past, cf. Phil. 3:6) and he is certainly not speaking of his present, given the timing of verses like Romans 6:17-18; 7:5-6; and 8:1-4 in relation to the summary in 7:25b.
4. Adam: A side issue is whether or not Paul has Adam in mind in 7:7-13. Perhaps most scholars think Paul did (e.g., Dunn above), although I myself remain unconvinced.
Ben Witherington would be somewhat typical. Here are some selections:
"Rom. 7:8 refers to a 'commandment' (singular) ... Only Adam, in all biblical history, was under only one commandment, and it was one about coveting..." (Romans, 189).
"v. 9 says, 'I was living once without/apart from the Law' The only person said in the Bible to be living before or without any law was Adam" (189).
"Sin is personified in this text, especially in v. 11, as if were like the snake in the garden" (189).
"Paul, then, is retelling the story of Adam from the past in vv. 7-13 and telling the story of all those in Adam in the present in vv. 14-25" (190).
I have a few questions about this line of interpretation. The first is of course why Paul never says anything explicitly about this. But even then, why does he use the word anazao in 7:9, "sin revived." Sin didn't revive in Adam's case, for he did not have sin before. In general, Paul's argument seems to be about someone whose flesh is under the power of sin, and it is unclear to me how this would apply to Adam before his sin.
5. Israel: Another side issue is the degree to which the "I" in this passage represents Israel. Moo has already given us a reasonable suggestion that Paul is speaking somewhat of the typical experience of a Jew and, thus, of Israel.
Tom Wright would be the one we would most expect to pursue this line of thought. Here are some selections:
"'The law' here, to repeat, is the Mosaic law, the Torah, and this is one of Paul's fullest discussions of it And those who are 'under the law' are, basically, Jews, and, by extension, those who attach themselves to Israel, i.e., God-fearers and proselytes" (New Interpreter's Bible, "Romans," 552).
"The change of tense [in verse 14] has to do ... with the change from the description of what happened when Torah first arrived in Israel, the time when Israel recapitulated the sin of Adam ... to the description of the ongoing state of those who live under the law" (553).
Wright thus seems to see the "I" of Romans 7 as a recapitulation of the story of Israel. Very clever indeed. But I wonder if once again Paul would have made this allusion clearer if it is in fact what he had meant. Nevertheless, the similarity of verses like 7:9 to 5:14 is tantalizing.
Conclusion: Paul's own autobiographical statements elsewhere make it unlikely that Romans 7 was ever a very good description of how Paul actually felt either as a Christian or in his pre-Christian days. In that sense I most agree with Krister Stendahl's analysis. It is possible that there are echoes of Adam in 7:7-12, but I believe these must be secondary if they exist. Paul is giving a theological run down of a Jew trying to keep the Mosaic law (and as Sanders points out, he picks a commandment whose keeping is less concrete than others and particularly susceptible to inner ambiguity). Perhaps the scenario is modeled on Adam, but it is not Adam. Similarly, Moo is surely correct to see the "I" as the typical Jew and thus see a connection with Israel. But Wright may go a little too far in virtually equating the "I" with Israel.