I agree with newenglandgenie. Both "OK" and "You bet" basically mean "Yes" but "You bet" is an enthusiastic "Yes!" ("Would you like another piece of birthday cake?" --"You bet!"), while "OK" is a neutral yes ("Will you pick up the birthday cake from the bakery today?" --"OK")
@luckyall You would only answer "You bet!" if you were enthusiastic or eager to do it. Maybe this client is your favorite (and fun) client or this client always gives a big monetary tip to whomever helps him. These are odd situations, but you could say "You bet!" in these situations. Otherwise, "You bet!" is a strange response in your scenario. Your best answer is "OK", meaning simply "I agree to do that", without any specific emotion associated with it.
After reading newenglandgenie's comment, I do agree that "You bet" can also be used to mean "Of course" or "Sure I can", without much enthusiasm, but this is a lesser used scenario. Boss - "Can I count on you to finish this project?" You - "You bet" (Don't worry. I promise you I can finish this project)