Hello BOA279. From what you say I believe the problem you are having is 'power no P.O.S.T' which means all components have power but the mainboard has halted startup because one of the tests carried out during 'Power On Self Test' has failed. This could be one or more of the following: CPU knackered or bent pins or not seated properly/ RAM faulty or not seated properly or contacts on RAM dirty or RAM socket faulty or dirty/ Mainboard faulty. As to fault elimination I personally would take out my cpu and have a look at it and check cleanliness of contacts on both cpu and socket then refit it. Next I would remove the RAM and look at the contacts along the bottom edge looking for a matt finish which would show oxidisation (bad contact). If so then a good clean with IPA (iso-propyl alcohol) and a soft brush might help. If not the very gentle use of a fibreglass pen will definatly ensure good conductivity. Next I would blow-out the ram slots with an air line/canned air or a gentle scrub with plenty of IPA and a brush, being sure to blow it very dry before power is turned on.
These are the main causes of 'Power on no POST' but it could also be any one of the peripherals attached to the mainboard such as hard drives, graphics cards, sata controllers anything really. To elliminate each component remove it from the motherboard and try in turn. I do this every day with laptops and find its usually the ram being dirty or the ram socket being dirty so start with that. If you need any more help give me a ring on 07769516161