My boys pc won't boot up, last successfully used to watch dvd.
Power seems ok. Blank screen so don't know for sure how far it gets but sounds as though not even getting to memory test.

Any ideas on likely candidate or could it be anything?

Need more information.
What OS
What Motherboard/Make
What results are from bootup (Beeps, Flashing screen, etc.)
Please be as specific as you can.

Dell machine about 2 1/2 years old, Windows xp pro,
boot up only seems to last a second or two, don't get as far as a beep, constant green power light but no flashing hard drive light, no screen activity at all.
First thought was graphics card because of screen but would that stop boot process?
Then considered memory but being a bit old I have none spare to swap.
May be three yrs old.
got it from brother in law and have not paid enough attention to know at this stage processor make or speed. sorry.

Sounds like it could be that a component has become loose inside. If the graphics card is loose and can't be detected, then your system won't boot. I'd expect to hear some kind of beep code to tell you that but it wouldn't be the first time that the BIOS has failed in its duty to provide a useful beep code to tell us what's wrong.

My recommendation would be to open up the case and inspect the components. Look at the graphics card and any other cards fitted into the PCI slots. Are they all sitting squarely and snugly in the slots or are they poking out more at one end? If necessary, remove the card(s) from their slots and then push them back in firmly along the full length of the copper connection. Then remove the memory modules and re-fit them, again firmly along the full length of the connection - make sure they 'click' into the slots and the retaining clips at the ends are locked in. If all of that looked OK, you'll need to remove the processor and fan and refit those as well - be careful with the processor as bending one of the pins on the underside can render it completely useless. You might actually want to test the system again before you go through removing the processor / fan (which can be a little awkward) just in case it is already fixed.

Let us know how you get on

P.S. For future reference, you can run the D-A-L.com Online Hardware Scan (there's a link in my signature below) to get the techy details about the insides of your system

DJNafey. Thanks for that. Have adjusted graphics and memory to no avail. Will try processor. If unsuccessful you recon 1 of these components to be faulty? How do you test which one?

Yes, it could be that one of the components is faulty. The only way that I've ever used to test a faulty processor, memory module or graphics card is to replace them with components that I know work or to place the dodgy components into a working machine and see if it still works afterwards. If the BIOS is emitting a beep code, that often helps point you towards the problem if you don't have a spare PC or spare parts. Memory can be tested using a specialist test tool but it wouldn't be viable to test just one PC - the test unit would cost much much more than buying all new memory for your system.

A quick check of the processor HEAT will tell you if its dead. To boot a PC you need 2 things, a Processor, and the BIOS. So, if you have Processor heat, then you have a damaged or corrupt BIOS file.