PSU Problem.

A while back I decided to build my own PC. I got an ASPIRE X-Infinity Case that had a 350W PSU in it. After I got all my parts I connected everything correctly and booted her up. It was pure joy to see my machine come to life...however that was soon ruined by the power cutting itself off after no more than 15 secs. I kept trying and each time I would boot it up, it would take less and less time for the power to shut off, to the point to where it wouldnt come on anymore.

I had come to the conclusion the PSU was probably bad and DOA. So I ordered a Rosewill 450W PSU. I just got it in today and I was very excited to get my computer up and running. At first it too did not want to come on which worried me about my motherboard being dead. After trying everything I could think of and making sure everything was connected correctly and snug, it still did not work. One last desperation attempt was clearing the CMOS on the motherboard. I tried it, and it worked. So the computer boots up and makes it a whole 20 seconds this time, then the power shuts off.

It's really urking me because this is the first PC I have built and I just wanted everything to go smoothly but it's not. I don't have the money to keep spending on PSU's or new motherboards. Can anyone help?

Oh and one more thing. I could get into the bios for a few seconds and it did recognize the ram and hdd. Thats about all I could see because it shut off too quickly. Would turning the voltage up on the back of the PSU to 230 do any good? Default is 115. I'm kind of scared I will fry something if I try so I need another opinion. My motherboard manual doesn't state which one to use.

Here are the specs:
Soltek SL-915GPro-FGR Socket 775
Intel P4 3.0Ghz Prescott
1Gb Corsair Value Select Ram (2x512)
36Gb WD 10k RPM Raptor SATA 150
NEC DVD R/RW Drive
Sony FDD