Imedia 1569 possible PSU problem?

Hey, first off just wanna say I'm not good with computers so I apologise if I sound like an idiot. and thanks in advance for reading all this.
I bought an Imedia 1569 a while back and was fine for about 6 months until it just wouldn't turn on, no power at all. After phoning PC world I was told to unplug all cables and hold the power button down. Did this and worked fine after that. Did it a couple more times but still worked until recently. The same thing happened but the solution didn't work. The light on the power button occasionally flashes when the power button is pressed and the fan turns slightly but that's about it.
On the advice of someone from another forum I tried switching the RAM and turning it on with only one but that made no difference.
I've had a look around this forum for similar cases but to be honest most of it went over my head.
I can't afford to get it repaired but I think it may possibly be the power supply because it's a power issue but I might be completely wrong. Do i just need to replace the PSU? If so where can I find a cheap, compatible one?
Information on the system: http://support.packardbell.com/uk/ [...] 137&g=1400
Thanks.

Thanks for the reply. I was just playing around with the wires and when I unplug the yellow/black wire connecting the motherboard to the PSU it starts fine. Does this mean it's the motherboard or could it still be the PSU?

Yes, the 4 pin cpu power cable and it runs fine with just that cable unplugged. Is there a cheap way to fix this?