Calculating the error in an estimate of <x^2>

Hi,

This may be a stupid question, I have some data in which I have a number of samples of a random variable (a component of a velocity distribution at a given radius in a star cluster)

They come from simulated data so there isn't an experimental error to speak of, what I am having trouble doing is estimating the error when calculating the mean squared of this, can anyone point me in the right direction of a reasonable method of estimating this?

Thanks,

S