Easiest way would be to buy a can of compressed air (which doesnt contain water..), or use a -small- air compressor. Be very careful when doing this as the optics and LCD's are very delicate.
All you have to do to get to the prism on this machine is remove the top cover you should be able get to the prism without needing to take the mainboard out.
It would also be a good idea to clean the filter on the underside of the projector. I have some pictures of the internals of this machine at http://c1.connxion.co.uk if you need some guidance to the prism.
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