boot - Random booting and rebooting

I have a very strange problem that I can not understand nor explain and is driving me crazy.

I have a Fujitsu Siemens Esprimo Edition P2411 tower PC. It has been running Windows XP from new quite happily for the past two years and I was not aware of any problems before this fault occurred.

Now it seems to almost randomly be powering on and/or off, firstly occurred as just a random shutdown of windows (as if power button pressed singly) but now seems to reboot in quick succession (at the bios screen) (sometimes passes BIOS, sometimes not).

I have tried various things, none seem to fix nor indicate what the real problem may be:

• Virus scan from avg usb disk, nothing suspect.
• Diconnect ethernet and ensure nothing in bios settings to auto boot, nothing apparent.
• Clean out with compressed air, lots of awful smoker dirt but dust all gone now.
• Replace power supply, problem remains.
• Complete disassembly and remove all components from case, dust down,
• • Rebuilt without connecting HDD/CDROM, problem not apparent
• • Added HDD/CDROM
• • Managed to attempt windows safe mode, reboots at giveio.sys every time
• • Problem back again
• Disconnect HDD/CDROM and power off, problem returns after 5-10 mins

There doesn't seem to be any pattern to the reboots that I can tell, I do not think the switch is faulty (buzzed "ok" with multimeter), sometimes it boots itself after 30 seconds, sometimes 5 mins off, sometiems hours. I can not think of anything software based that could cause this and the tower seems quite clean from dust now.

Can anybody suggest any course of action or shed any light on this problem that is driving me up the wall.

Many thanks.

I can almost guarantee this is a RAM issue. But there are other possibilities of course. Since it happens outside of your OS as well, we can rule out a software problem.
Start with a single DIMM of ram in slot 1 - test
Move the DIMM to slot 2 - test
Move to any other slots - test each one. If you get no restarts, set this DIMM aside and repeat with all your DIMMS. Most likely you will find that one of the DIMMs doesn't work in any slot. You may also find that all DIMMs fail in one single slot.
This will pretty much tell you if it is a RAM or Motherboard issue. You can use MemTest86+ to stress your ram for testing purposes. It is also available on the UBCD along with a lot of other diagnostic tools.

It sounds to me like a matter of a Computer simply getting old and wearing out. Based on what you have tried, about all that is left is either the CPU, RAM or Mother Board.

You could also try booting and Linux live CD and making sure you experience the same issue there. If you do, you can be confident that it is a hardware issue.

A PC's 'self preservation' can be misleading. I've had seemingly random reboots boil down to overheating, though that can vary based on room temp as well as what's being run. You can easily go hours without heating up the cpu, gpu, or ram - whatever's worn out its resiliency.