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Intel Pentium 4
3gb RAM (2 + 1)
Intel I945P/G Chipset
GeForce 6600 GT
Windows XP SP3

During computer work it sometimes (about 3-5 times per day) reboot itself. It doesn't matter if it's under heavy work, or left idle. The "autoreboot on error" option in system configuration is disabled and there are no bluescreens in system log.

I also saw something weird on POST screen - single random letter was printed on screen with wrong color (white instead of gray). I saw that at least two times, it was different letter each time, and this not happen during most boots. EDIT, additional info:

I've checked temperatures: it's below 65'C for CPU right after reboot, other periphials have even less. I also checked power supplier's voltage output, it is withing safe values for +3.3V, +5V, and +12V. There is something really strange on -12V (it jumps randomly from -7V to -13V), but I've found over Google that this line is usually unused and I should not worry about it.

For now I suspect the motherboard failure, but it's more like I don't have any other idea to check.

EDIT2: I've run memtest for 10 hours, 12 pass, no errors, no reboots

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Follow the instructions for Windows XP without a BSOD error log or a crash dump we are unable to help you with your problem.… – Ramhound Jun 4 '13 at 23:05
@Ramhound there are no BSODs in my case. There are no STOP errors in Windows Event Log, the option "autoreboot on error" is disabled and it still reboots randomly – PiotrK Jun 4 '13 at 23:13
If you cannot provide us something then we cannot help determine the cause of the crash. Did you enable the crash dumps. – Ramhound Jun 4 '13 at 23:31
@Ramhound I've already had mini crash dumps enabled (via system properties), but still C:\windows\Minidump is empty and I strongly believe there are no BSODs in my case which are required to generate crash dumps – PiotrK Jun 4 '13 at 23:59
up vote 1 down vote accepted

The fact that there are no logs and "autoreboot on error" is disabled indicates that it's almost certainly a hardware issue. Of course, just about any faulty piece of hardware could cause a reboot, but I'd start with the PSU, Motherboard, etc.

If it doesn't reboot during a memtest, it's likely because memtest doesn't use the exact piece of hardware that causes the issue, or doesn't use it in the way that causes it to fault. Since memtest only uses the absolute minimum hardware resources needed to test memory, it could still be pretty much anything. Even the PSU is still suspect as memtest won't put nearly as much load on it as an actual running system does.

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It was a faulty PSU, I've replaced it and the problem is gone :) – PiotrK Aug 14 '13 at 8:06

The two reboot causes I've come across personally are recently updated device driver files, which I never would have believed had it not happened to me, and overheating. Sometimes the BIOS will monitor & report fan failures if one of those is malfunctioning and causing the Power Supply or CPU to overheat, so you might look in there as the computer boots up (usually F2 or DEL key to enter it). Good Luck!

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There is no new devices added to this computer for ages (except additional non-system hard disc about 4 months ago) and there is 80'C warning set up in BIOS which never fired. It was also dust-cleaned recently. I checked temperatures just after reboot - the largest was about 65'C on CPU – PiotrK Jun 4 '13 at 22:38

Could be malware causing your random reboots.

Also, failing hardware such as graphic cards can cause random reboots sometimes. May also explain the teensy bit of graphic corruption you saw on one boot - as that video was coming through the graphics card, right? You may try removing your graphics card and see if things improve.

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