[susan53]

The second one is weird isn't it? It seems to say the opposite of what it means and yet it does actually seem to give a more positive tone to the sentence than the affirmative. It happens a lot after if/whether (ie the non-conditional use of if) - eg I wonder if/whether they wouldn't prefer Wednesday.

For me the affirmative leaves the question open : I'll see if I can help or not - It gives the idea that I don't know, or haven't decided. The answer may be yes or no.
I'll see if I can't help, on the other hand, implies that I want/intend to help as long as nothing unpredicted stops me.