If God is willing to prevent evil, but is not able to?
Then He is not omnipotent.
If He is able, but not willing?
Then He is malevolent.
If He is both able and willing?
Then whence cometh evil?
If He is neither able nor willing?
Then why call Him God?
The second, written by Hume (and my first exposure to the problem of evil), is a stronger formulation of the same argument:
"[Gods] power we allow [is] infinite: Whatever he wills is executed: But neither man nor any other animal are happy: Therefore he does not will their happiness. His wisdom is infinite: He is never mistaken in choosing the means to any end: But the course of nature tends not to human or animal felicity: Therefore it is not established for that purpose. Through the whole compass of human knowledge, there are no inferences more certain and infallible than these. In what respect, then, do his benevolence and mercy resemble the benevolence and mercy of men?"
I've yet to hear someone refute this without relying on the tired cannard of "Free Will." After all, even if we have free will, God should still be helping us if he is capable, omnipotent, all-loving, blah-blah-blah...