Computer start up problem. PS?

[thermostat]

I am having trouble getting my computer to start. It has been fine. I have generic 400 watt power supply and all of a sudden, after hitting the power button, the lights and power comes on for about a second and then shuts down. After a few seconds, I try to start again and it might or might not start. IT does start finally. I have disconnected components one by one and does not seem to matter. I am assuming it is the power supply as more more is needed to start up obviously but there are no problems once running which baffles me. All voltages are fine by MBM 5. Any ideas?

 

[WastedYouth]

i had this problem.. my PSU was shot... got a new one and a new CPU cus i thort that was bust but it was just the PSU... have you thort of trying another one?

but then again u mention that urs sometimes starts but mine didnt... maybe u got a dodgy switch??? sorry cant be of anymore help

 

[thermostat]

After trying some more things, I realized I added a new 80 G hard drive in place of a 40 about a month ago. I put the 40 back in and lo and behold it boots up everytime. After checking the power requirements, the 80 is about double. So I believe my problem lies in the power supply not being about to supply enough startup amps. If I can get past that everything is fine. So I guess a new power supply is about to be bought. 400 watts is not always what it is claimed. NAme brand here we come. Any suggestions?