Both are standard expressions for asking "how are you?", "how are you doing?".

For "mitä kuuluu" the answer is "hyvää" (fine).
For "miten menee" the answer is "hyvin" (also fine).

HTH

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Note added at 2003-11-16 10:25:47 (GMT) Post-grading
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Mmhh what I wanted to point out is that mitä kuuluu and miten menee are better solutions for a simple, small-talk \"how are you\" question, whereas kuinka voit is more used when asking how a person is feeling, maybe because he or she is not looking fine or something. In Finland, when you meet someone, you are likely to ask mitä kuuluu and not much likely to ask kuinka voit. At least, on the basis of my knowledge and direct experience. That\'s all :)
Don\'t know why you pointed that out, but the question by the asker HAS a question mark.