I have recently decided to buy a new 160G hard disk for my computer.  HOWEVER I found that my current 300W PSU did not have any spare connectors for this add in.  So I bought an Antec Truepower Trio 430W PSU.  I removed my old power supply and replaced it with the new one, plugged everything in, except the new hard DRIVE...just to test it.  Turned the power on, everything started and then died about 3 seconds later.  I was perplexed, I hadnt even turned on the soft switch on the chassis, then it did it again.  The computer turns on all the fans start going, then dies about 3 seconds later.  The cycle continued until i turned the PSU off and replaced it with my old one.  The system then worked FINE.  I was advised to update my BIOS so I did but the problem still exists.   I am wondering is it a hardware clash or is there a fault in the PSU itself.  Thanks for your help in advance, David.  SOUNDS like the PSU is either faulty or not compatible. I would return it, and buy one of these:

http://www.microdirect.co.uk/(2165)Hard-Disk-Drive-Power-Splitter-5-to-2x-5-RB511.aspx

if your new disk is SATA, you'll need one of these too:

http://www.microdirect.co.uk/(2931)Serial-ATA-power-converter-cable.aspx
I have contacted the supplier and will return it shortly.  Thanks very much for your advice.  Just for closure.  Returned PSU to my supplier who then tested the system.  They didn't find anything in the preliminary voltage test but gave me a replacement (same model) to try.  Brought it home plugged it in and it works like a charm.  Still don't know what the problem was but it was definitely that particular unit.  Thanks again.  thank you for the follow up. it contributes greatly to the forum.