Google "The Dead PC." and you find the usual method of stripping it down to as few parts as can be.
But the no power at all points strongly to the PSU.
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=c01957794 has more specs and it looks like a standard supply. Google "PSU Green Wire Test" if you don't have a spare PSU.

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