[chas]

You might know this, but I want to state it at least for the benefit of people reading this without much signal processing experience. Just zeroing like that really isn't the best idea. Square waves (that are 1 for signals that you want and 0 for signals you don't want) inverse Fourier transform to sinc functions[1], so if you are filtering just by chopping things out, it will greatly distort the time-domain system because the inverse Fourier transform of the filter and the signal get combined (convolved) in the time domain.

[1] http://en.wikipedia.org/wiki/Sinc_function

[msandford]

Yeah exactly right. If you don't make it smooth in the frequency domain it won't be smooth in the time domain either.

[oakwhiz]

Wouldn't the result would be as smooth as the FFT's window function?