OK, I have tried my best - thought i would be cheaper to buy all my PC parts individually and build the whole thing myself but keep coming across problems:

Here goes:

Had existing:

ATX Case:
PSU
Hard-drive
Monitor

Bought new:
MSI motherboard (DDR, AMD, etc) (ebuyer)
DDRRAM (ebay)
CPU with fan (Ebuyer)

Plus a video card and case fans etc..

Connected everything up and nothing happened, so tested PSU and found it was gone (no idea why) so found an old PSU (i think it's from an AT case as it doesn't fit in mine! I'm a bit clueless so might not be!) but connected this up and YES!! PC turned on!

Alas, screen does not switch on, I here no beeps and the although the fans spin, not much else happens - except the HDD red LED comes on constantly from start and the hard drive does not spin (I have a spare and checked that and it doesn't work either!)

Firstly,

thought this might be the PSU - would it matter what type of case it is supposed to fit in?
is the CPU likely to be gone? - if it is, would this sort of thing happen?
DDRRAM (if this is faulty - as it's from ebay) how would I work out if this is working?


Any help at all would be brilliant!!

Have you pulled the HDD and DDR ram from the MB so that it can attempt to boot?

An under-power supply will cause the system not to power up fully. Minimum PSU rating should be 450W for a modern system, some can get away with a 300W.

Pulling the HDD and Ram will allow the system to attempt to BEEP that there is no memory or HDD connected if the processor and bios still work.

Thanks, i will give it a try tonight. I'll have to buy a new PSU any how so I'll get hold a 450w one and give that a try.

Cheers

AphJN's "back to basics" approach is very logical - ruling out as many possible factors will lead you to the cause of the problem much more quickly.

However, there is something else that struck me when I read your post. Firstly, if it's an AT power supply, it will not have the right power connectors to plug into your new motherboard, so it must be ATX. Having said that, though, it still might not be the correct type if it's a particularly old one. Pentium 4 processors (if that's what you bought) require a "P4 PFC" power supply. If the PSU is from a Pentium II or Pentium III system, it's probably not compatible with your processor.

Any brand new one these days ought to be the right type but it'll be worth double-checking.

Thanks, I thought an AT supply wouldn't fit so must be an old ATX supply. (I think I got it from a PC with a win 98 system)
I'll try taking everything out individually one by one as that seems the most logical approach. My only has problem has been that as I don't have any spare CPU, RAM etc, it'll be difficult to isolate if it turns out not to be the PSU! I'll get a new one anyway and hope for the best.

Cheers