When I press the power button on my desktop, it does not power up completely. Before I press the power button, I see lights on the motherboard. Everything is normal. On power button press, the fans on the cpu, graphics card and motherboard start to spin a little for a second or two and then they stop. No beeps during this process.

It has been doing this for a while now but it used to start up after some trials. Once it starts up, I have NO issues at all like random shutdowns so it is not an issue with OS.

Update: I left the desktop off for a few days and it started.

I'm just guessing here but it seems as if the PSU (Antec TP2-550ATX) is dying out and does not have enough power now - just a guess. It's an old desktop assembled in 2005 but I have maintained it well.

Update: I always keep the desktop running and I never shut it down. During updates or manual restarts, it powers up without issues. I wonder if this sheds lights on the issue.

Any idea how I can narrow down the issue? ex: if I can find if the PSU is dying etc. I'd really like to fix the issue. Please help. Thanks.

Below is the complete configuration.

DFI LAN-Party UT NF4 Ultra-D 6/23 {6.70},
Evercool EC-VC-RE 41/47C,
AMD Opteron 170 2.0GHz {1.3.2.16} 1.312V 36/41C, 
ThermalRight SI-120, Panaflo 120×38mm
OCZ Platinum 2×1GB 200MHz 2.66V 3-3-2-7 1T
XFX 7800GTX 256MB 475/1250MHz {91.31}, 
Zalman VF900 Cu led 41/56C
WD Caviar 320GB 7200RPM 16MB SATA 3Gb/s
Antec TP2-550ATX
Antec P180
WinXP sp3
Logitech MX310
Razer Mantis Speed
BenQ FP91G+ 19" LCD 8ms DVI
Creative Audigy2 ZS {4.42}
BenQ DW1640
Logitech z-5300e 5.1 280W

Legend:
Driver versions: {}
User settings: []
Voltage: V
Wattage: W
Temperature: C (Celsius) min/max