shortcircuit is a freeware software sampler that would do what you are asking. what DAW are you using? they might have a built-in sampler you could be using for this
apple really stepped up their editing in logic 9 (flex time, anyone?), spend as long as you can getting to know the ins and outs of it and you will fall deeply in love. you have some built in samplers:
EXS24 - good sampler, not too complex
Ultrabeat (personal favorite) - crazy sampler, bit more complex, but with a better GUI than the EXS imo, makes it a lot easier to put your samples where you want on your midi keyboard.