SAFETY FIRST!!! UNPLUG YOU PC FROM THE MAINS. Never take a Power supply apart unless you know how capacitors work, they hold power and it hurts or could be fatal.
I know most of you can build a computer, but here are a few problems I have had in the past. Before going out and renewing hardware use this as a rough check list, it could save you money. So you can send it me
No Boot: screen empty: Bios setting changed via a program. Incorrect fitting of Memory or processor. You need fresh paste on all new processors now don't try to use the old paste it won't work as well (If it does work) Memory: When changing memory think Environment, is it dusty? do the contacts want cleaning?
Clean them anyway. I have 3 brand new paint brushes to clean my pc out when ever it needs a new part. Never use old paint brushes or any type of cloth. If you have a hairy animal cat , dog ,older sister give you pc a clean inside about every 3 months.
Pc cutting out/turning off. This error cost me £54 I didn't need to spend. It was the connector from the psu to the MB. Some of them have to be pushed home even though it looks all the way in and the clip is on "double check".
Cables catching Processor fan, when you have been inside your pc use tie wraps the ensure the cables can NOT move and touch any moving parts i.e. fans on processor or video card.
Memory not all the way home or not clean, also check for bent pins on the board.
OS not starting: try booting in safe mode 9/10 times it's the os gone faulty. You should get a message if the hard drive is disconnected or not working. Check the cable and connectors,dirty or bent pins. Is the connector all the way in and the right way round? That go's for the power to the hard drive as well.
If you have a secondary had drive in disconnect it then power up, if it works then check secondary as described above. If other people live in your home especially young children check for objects pushed into ports like USB (Yes my nephew did that to my bro's pc) If you have no sound check first the head phone socket on your speakers and pc have the kids or mum and dad pushed anything in?
I now have what I call my test station, scrap parts built up in a tower, cost me about £10 from the re-cycling dump. test hard drive memory psu and processor (amd the best)> If everything works on the test station then its possibly the MB. Also remember since sp2 update winxp will not work if you put the hard drive on a different MB it won't boot.
I think that's all for now if I remember any more I'll let you know lol