Is it possible for a PSU to power up the fans, yet still be defective sometimes, and not produce enough power to boot the machine?
HDD Seagate ST340016A 40GB + Radeon 7000 32MB VGA (PCI) Card
All PCI empty
[have a lot of problems, trying to eliminate 1 by 1]

ANY ADVICE APPRECIATED

HEC 250AR-PT 250W
Total output: 250w
Combined output: 3.3v + 5v: 150w
Max Current: +3.3v=20A, +5v=25A, +12v=13A
Safety Certificates: TUV, CE, NEMKO, CB, UL, CSA
EMC: Complies with CISPR 22 230 VAC operation.
Intel V.2.03 ATX Specification
Size: H 86mm, W 150mm, D 140mm

SiS 655FX

P4S55FX+

CPU

- Socket 478, Intel Prescott Intel Pentium 4 Celeron
- FSB 800MHz/533MHz

Chipset

- SiS 655FX+964L, Supports Hyper-Threading Technology

Memory

- Supports Dual Channel Technology
- 4 x DDR DIMM slots
- DDR 400/333/266

hi its probly a bit late but i had the sam thing have u tried a mor powerfull psu

Look at all the components google them around and find the consumption of all of them in power. To see if you're PSU can cover it. It would be best if you're PSU cleared the consumption by a lot instead of just clearing it to put less strain on the PSU. Im not saying you should buy a 1300w PSU to cover 250w of power need or anything like that steep but make sure you clear it be a healthy amount.
Check while googling you're parts to make sure they are compatable if they are not. When started it will power up and activate the fans but then fail.
As on boot you're computers BIOS trys to synchronise all you're computers components (get them communicating) ready for the windows system. to be fully functional if they are not compatable you may struggle to boot successfuly.