Hay umm i think i can give you a vage description of what you can do. If you have heard of the jumper effect you can use a part of that for the disapperance and then use particular for the sand. If you want a link to a good jumper tutorial and how to make good looking sand with particular just reply back.
The first thing you have to do is to separate the person from the background. If you have already shot the footage then this can be very difficult. If you haven't shot yet you can shoot a clean plate with the person not in the shot, then shoot a plate with the person in the shot or shoot with a green screen. (Plate means shot).
Then you separate have to cut out the person for the number of frames you want the effect to last and apply an effect to the layer with the actor. Shatter (Effects>Simulation>Shatter) is one way to do it. You adjust the physics so the particles blow away in the direction, you use a noise layer to create the custom shatter map. You use a gradient to control the timing of the shatter and maybe some turbulence from Turbulent Displace or Fractal Noise to distort the shatter map, then monkey with the settings until you get what you want.
A quick Google Search for Custom Shatter map will bring up this tutorial. I'd never seen it before, and I didn't watch the whole thing, but it should get you started. Just play with the physics settings to get the particles to fly away. I'd start with the gravity direction and inclination.
Thanks everyone for the answers. I would probably use a still that I could isolate in photoshop and then bring into AE and have that picture sand dissolve.
And yes I would appreciate a link to the jumper tutorial.
Thanks again Roman