When switching the power on at the power outlet, to which my computer is connected, the computer starts automatically without having to press the ON switch on the computer case. Shutting down the computer works correctly, as the machine automatically powers-off and does not start again until I press the ON switch. The computer is running Windows XP.

Some computers have the option to automatically restart after a power loss has been detected. Even when your computer is off, but is connected to a live power source, there is still a small voltage which is supplied to the computer motherboard (on some computers you may notice an LED on the motherboard which stays illuminated even when the computer is powered-down). Since you are turning the computer off at the power outlet, there is a loss of power to the computer. When the power is returned, the computer detects the loss and restarts. There should be an option in your computer BIOS to enable/disable this feature. Only proceed with the following procedure if you are comfortable in editing your computer BIOS, since changing the wrong settings can cause serious consequences. Should you wish to proceed, restart the computer and during the power-on self test (POST) you should see a message similar to ‘Press DEL [or some other key] to enter setup’. When you see this message, press the nominated key to open the BIOS setup. You will need to look through the BIOS setup pages, for a setting like ‘Power Loss Restart’ (or alike). Once you have found the setting, set the value to disabled. Then, close the BIOS and save changes.

Should this not fix the problem, you should check for other settings in the BIOS which could be causing the computer to power-on. For example, you may find a Wake-On-Modem setting in the BIOS which is enabled. If you have a modem connected to your computer, when you switch on the power at the power-outlet this may switch on the modem and the modem could be sending an initialisation signal to the computer causing the computer to power-on. Likewise, if the Wake-On-LAN setting is enabled in the BIOS, when the computer receives power the internal network card may receive/send some initial traffic from/to the network, causing the computer to power-on. So, go through the BIOS and disable any settings relating to waking for particular devices.

If you continue to have problems, there is a possibility something is wrong with the power supply or motherboard in your computer. In this case, you should consider taking your computer to a computer repairer/technician to check the components to make sure there are no potential safety problems with the setup.