Low power supply voltage or intermittent loss of voltage can cause a reboot. Set the BIOS to have the PC remain off instead of turning on when power is applied. You can then tell if you have a problem with power because the PC will remain off instead of rebooting.

Martin wrote:
>
> I have a problem with a home-build PC that's been running XP with SP2
> flawlessly since Aug 2005.
>
> In the past two or three months it's developed a problem where it
> randomly just reboots itself.
> To begin with it would only reboots very occasionally so the problem
> was not dealt with.
> In recent weeks the PC has taken to rebooting very often so i was given
> the job of fixing it.
>
> First thing i suspected was CPU overheating - BIOS reporting temps of
> approx 65C at idle just minutes (seconds even) after booting from cold.
> I removed the CPU's thermal pad, cleaned both CPU and heatsink with
> spirits and applied a small blob of thermal paste.
> CPU temp now averaging a much more healthy 50C on average - the HSF is
> not a performance model, just an el cheapo, so i suspect that's about
> the coolest it'll run without a better HSF.
> Anyway the reboots continued after the CPU temp treatment so i rules
> out CPU overheating.
>
> The reboots happen anytime - either when booted into XP or just in the
> BIOS setup mode - so i ruled out any software problem.
> (We did successfully clean install XP 7 days ago, this was when the PC
> wasn't showing any signs of rebooting - needless to say the problem
> continued).
>
> I removed the AGP card, cleaned the contacts with a pencil eraser, and
> reinstalled it - still the reboots occur.
> (The PC has no PCI cards to inspect).
>
> I removed both memory modules and reseated them - still the reboots
> occur.
>
> Only components left to suspect (other than the motherboard) were the
> two hard drives and the CD-ROM.
> CD-ROM was prime suspect as it's ancient!
> CD-ROM has been disconnected (both IDE and power cables unplugged) for
> the past 5 days - still the reboots occur.
>
> The motherboard's BIOS is the last but one update available for it -
> the latest BIOS update states that it includes a fix for large hard
> drives, one of the PC's hard drives is a 200GB drive so i downloaded
> the BIOS update and decided to update the BIOS.
> The PC has no floppy so i dug out a CDR that i'd made years ago with
> the WinME boot floppy as it's image.
> With that i could boot to DOS and update the BIOS i hoped.
> (The BIOS update and BIOS updater being on a FAT32 partition that i
> hoped to be able to access after booting from the WinME boot CD).
>
> I reconnected the CD-ROM inserted the WinME CD (the PC was booted to XP
> at the time) and as soon as i closed the CD-ROM tray the PC rebooted.
> It then went into a loop rebooting, getting to the part of the BIOS
> screen where it scans for IDE devices and then as the CD-ROM spins up
> it'd reboot.
>
> I removed the WinME boot CD and it booted ok but after less than 15
> minutes started to reboot.
> I disconnected the CD-ROM and again reboots happen.
>
> I then disconencted the 200GB drive leaving it to run on it's 40GB boot
> drive - within less than 15 moinutes it'd start rebooting - so maybe
> the BIOS update with the large hard drive fix wasn't going to help
> anyway.
>
> I'm now suspecting the PSU is the cause - a power drain surge as the
> CD-ROM spins up may explain them reboots. The fact that the reboots
> happen with the CD-ROM disconnected indicate that the CD-ROM itself is
> unlikely to be the cause.
>
> The PC is a homebuild with a cheapo case and an el cheapo unbranded
> 300W PSU.
> It's worked perfectly from August last year up to about 10 weeks ago
> and the reboots happen more and more.
>
> Here's some technical details about the PC:
> Foxconn K7S741GXMG motherboard.
> Sempron 2400+ CPU.
> 2 x 256MB DDR333 DIMMs.
> Connect 3D Radeon 9550 AGP card.
> 200GB and 40GB hard drives.
> CD-ROM.
>
> BIOS 'PC Health' screen reports these voltages:
> CPU core - 1.58v
> +3.3v - between 3.21v and 3.26v averaging about 3.24v
> +5v - 5.15v
> +12v - 12.32v
>
> What do you all think then?
>
> The PC's owner hopes to add a DVD writer sometime soon so i reasoned
> that a new PSU would not only (hopefully) fix the reboots but also
> prepare it for another device that'll be more watts of power.
>
> Thanks for any help.
>
> Martin.

--
Mike Walsh
West Palm Beach, Florida, U.S.A.