This is known as SFFS (spontaneous fuse failure syndrome) - there will never be any warning prior to the sudden failure and an asprin will probably not help - well certainly not help the microwave.
This link might help with the interior anatomy...
http://forum.appliancepartspros.com/microwave-repair/1406-need-locate-microwave-fuse-location.html
Take care as dangerous radiation and very high voltages lurk inside the body cavity.
Good luck...
Peccavi
December 2009
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