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