PC Shutting Down Constantly

[miw28]

I have search on this board for shutdown problem and have read about 100+ postings, but could not find that have my problem...SO hopefully someone can help me..

My PC would go to shutdown and power off automatically while working on any open program. Sometimes I can go for 2-4 hrs without the computer shutingdown, and sometimes only 30 minutes before it shutsdown. I am on a WIN2K Pro SP4 with all updates and security patch installed. No virus blaster, no worms, no ad-wares, etc...because already scanned for all these stuff; have configure firewall and router connection w/DSL.

When it shutsdown, a message "Window is shutting down" appears and the computer power just goes off so fast as though I switch the power off. Then, 5-15 minutes later, the computer would try to power up by itself without me doing anything. On a daily basis (every morning), the computer would try to power up but could not go to the Windows start-up screen; instead it would run to the BIOS screen, and a message on the BIOS would say the computer hang during boot up because of "improper cpu speed....now it's in safe mode blah...blah..blah..

I have already change a new power supply, have tried swaping RAMS around, have taken off both panels of the computer, turn on the airconditioner in the room, updated all the drivers for all the AGP, Sound card, Network card; disconnect my APC Backups Pro power supply, check-out all cable connections, scan for harddisk errors, defrag harddisk, updated the BIOS on the mobo

I think I am having a hardware problem instead of software, because I even tried installing a New harddisk with a fresh installed of WIN XP Pro SP1 with no softwares/updates installation yet, and still the computer would shutdowns after a few hours or sometimes few minutes loggin.

Please help, is it my CPU or Motherboard that is going old?
What else can I do to diagnose and get to the bottom of this problem?

I have disable the Auto Restart on the System, BUT not sure what this would help me show? Because on my Event Viewer/System Log/Security Log, everything looks normal. It doesn't show any software or hardware failure or anything wrong. Before the PC shutsdown, there is no error message at all. Even when it boots up again by itself, no error messages.

And if I disabled the Auto Restart, then why Win2K still try to power back on after it shuts me down?

my comp specs:
PIII 500Mhz
Asus P3B-F, Intel Chipset 82440BX
RAM Visipro 256MB-768MB PC100 SDRAM
NVIDIA Geforce2 MX200 32MB
Creative Soundcard VIBRA 128
Creative CDROM Infra6000
Sony CDRW 16X10X40
2 Hard Disk

[doctorgonzo]

Do you have options in the BIOS for Wake on Lan/Keyboard/Ring/Etc.? If you do, make sure they are disabled. They may be malfunctioning.

What brand is your PSU?

[miw28]

Yes, I have already disable all those wake on lan/keyboar/ring and even in the Control Panel/Power Options to be Always On/No Screen Savers/turn-off monitor/hard disk to never/ system standby to never/ disable hybernate, etc...

Yes, I have also turnover anything that could have initiate the system to go off..

What I can remember is that I got this problem after installing Windows Service Pack 4 AND also after I change my sound card. The old sound card was broken; and then I installed a new sound card. But I don't know what caused this really since I also tried the new sound card on a different PC and it works just fine. I also installed SP4 on different PC and it didn't have any problem.

Could it be the RAM or the chipset of the motherboard?

I am assuming PSU is power supply? My Power Supply is ENLIGHT, same manufacture of the casing (400W)