Each night my computer automatically turns on at midnight. I have looked through the BIOS but cannot find any timers or anything else that could cause this behaviour. The computer is running Windows XP Service Pack 3.

Without knowing the exact specifications of your computer (such as brand or, in the case of a custom-built computer, the motherboard model) it will be difficult to provide step-by-step instructions for troubleshooting this problem. However, we can explore the various potential generic causes of this issue which will hopefully point you in the right direction. You are correct that the most common cause of the issue is some power management setting in the computer BIOS which has been configured to start the computer at a particular time. It seems that you have already gone through the BIOS searching for such a setting, to no avail. However, I would recommend that you download the manual for your motherboard and double-check, just in case there is a setting which you have missed.

There are also several other settings in the BIOS setup that could be causing your computer to boot, such as Wake-On-LAN (WOL) which is designed to automatically boot the computer when the network card receives a special WOL data packet, although this would then beg the question as to what device is sending the computer such a packet (but we will deal with this question shortly, should we find this is actually the cause). To determine whether some kind of external peripheral is causing the computer to startup, try disconnecting everything from the computer, with the obvious exception of the power cable. If the computer no longer starts-up under its own accord then one of those devices or connections is causing the automatic boot-up at midnight. In this case I suggest that each night you disconnect one device and check whether the problem happens. Alternatively, you can try a (faster) divide and conquer approach by disconnecting half the devices and checking whether the problem persists. If so, you know that one of those devices is the cause. If not, you know that one of the devices in the other half is the cause. You then repeat this procedure (continually dividing the halves) until you reach a single device which is the root cause. Once you have found the problematic device you will hopefully be able to do some more research to determine why that may be causing the problem and how it can be fixed. You can also contact me for further advice if needed.

However, should you find that the problem still occurs after disconnecting all the cables then to narrow down the problem I will need further details of the specific computer configuration (particularly the brand name and model, if a brand name computer, or motherboard brand and model, if a custom-built computer). If you could provide these details I can investigate the problem further.