Can you give us a full spec of your system?

Does nothing at all happen when you press the power button?

Does your machine have a video card as well as integrated graphics on the mb? try pulling the vid card and seeing if the machine can run with the IGP.

do suggestion above... and have you tried resseting the CMOS batt.. or either disconnecting the power cord from your AC line for about 5 mins.. check if it will help

Thanks for the reply and apologies for not getting back sooner.
Problem is the spec of the pc is inside it and all I can tell you is that it is Fujitsu/Siemens scaleo P series, 240GB HDD 1GB RAM. I use an external Behringer soundcard and I don't know what make the motherboard is or the graphics card. (The ones that came installed)
Sorry for being so vague about the spec.

Yes, no power at all.

Cheers.

Thanks for the other replies. I will try those suggestions and get back to you. Thanks again!

Then your sys has a discrete NVIDIA graphics card [ no integrated graphics on the mb].
It may have a problem.

Don't know if this will have any bearing, but i had a similar problem that appeared random. I sent back several nvidia cards (ultra 6800 oc) and they appeared to work for a short time and die again. After the last return they sent me a 7800gs card or something. Did it again, just shut off. So in desperation I opened the case prepared to undo all the cables and things installed and inspect them. My 12 pin connector from the power supply to the motherboard was melted together. I managed to get them apart and saw that some of the pins were black so i cleaned everything off and put them back together and it worked. Occasionally id have to reclean the pins until i switched power supplies. No problem since. I know its a bandaid but I just wondered if the video card could have drawn so much power that is melted the connector or something.

Sometimes poorly made connectors can give a minimal contact to pins... the currents involved then get to a current density int eh contact area where the temperature increase in a tiny volume of metal can be enough to cause sparking [metal burning] and welding... the resulting oxides don't help the connectivity, and so it goes. A lot of pins and connectors have a smear of gold electroplated on the pins to reduce oxide formation.. but they still require a decent conatact area between them.

Thanks a lot for the replies. I tried taking out the graphics card but no change. I have to get a CMOS battery today to check that. I'll check all connectors later aswell. Incidentally the MB is an ASUS and the graphics card is as you said is an NVIDIA G-force,
Thanks again, I'll let you know how it goes. Cheers!