The original comment about the movie here. Spoilers below.
I've read this and this about the movie and the Wikipedia entry. If you haven't seen the movie or read the book you can get a good idea of them from Wikipedia.
I feel kind of foolish for asking what this book means, when one of those posts says that the movie is didactic (and the book apparently more so). The biggest theme in the book/movie is pretty obvious: that mankind is getting progressively more evil. Hence the title, No Country for Old Men. Old men need tranquility not drug deals and endless violence.
I do have a couple questions about that theme. Why is the book set in 1980 (ish)? Presumably we are more evil 25 years later (although, really the crime numbers don't bear that out). Bell criticizes the culture of the time (kids with green hair and bones in their noses), but a lot of those criticisms seem dated. So I guess I'm not sure exactly what the commentary is; is it that the world is getting worse? are we just to believe that evil is always with us? or is the commentary subtly ironic and meant to mean the opposite? If it's meant to be ironic, it's probably a bit too subtle.
Chigurh is obviously a symbol/embodiment of evil, and probably Satan, romping the countryside and killing almost indiscriminately. His murders aren't completely random, however, because he does, as one character points out, follow a code. He is a man of his word. When he promises Moss that he will kill his wife, he has to follow through. He also returns the money he retrieves from Moss to the drug dealer, almost like he respects some property rights. He also flips a coin at certain points to make decision about whether to kill certain people or let them go. She he respects the laws of chance and/or fate.
Moss and Bell are both sympathetic characters, but I think show the limits and flaws of humanity's resistance to evil. Moss is killed ultimately because he takes someone's money, albeit drug money. All his attempts at good--going to give the dying Mexican a drink, giving the hitchhiking girl a ride--end up disastrously for him and the intended beneficiary. One thing the book makes clear that the movie doesn't is that while Bell is decorated WWII vet, he actually abandoned his position when he maybe should have held it. Bell is embarrassed about it although the portrayal of his cowardice is unconvincing. Still I get the sense that at the near encounter between Bell and Chigurh at the hotel that Bell avoided the contact. Also, he retires from the job in part because he feels outmatched by Chigurh's. Maybe this is because I knew what would happen before hand, but I feel like McCarthy telegraphs that Bell has no chance of catching Chigurh. Bell represents a flawed and outmatched good.
The movie is conservative, not just because Bell is a sympathetic conservative character, but mostly because of its portrayal of evil. In this story, the devil is real. The book shows, and Conservatives believe, that humans are inherently flawed. Conservatives believe evil exists for evil's sake. Liberals, in contrast, believe in the perfectibility of humans, and do not believe in the kind of evil that Chigurh represents. But the book makes clear that Chigurh's actions can't be understood as anything but a desire to do evil.