When cold booting the computer (i.e. starting from a power-off position) a BIOS error always appears, notifying the computer settings are wrong and giving me the option to run with the factory default settings or enter the BIOS set up. If I enter the BIOS setup the date is 2002 and the CPU speed is incorrect. The computer is a two-year old Pentium 1.7 GHz running Windows XP Professional with 512MB RAM.

It appears that your CMOS battery is dead, or is in the process of dying. Relevant to this explanation, there are two types of ‘memory’ on the motherboard: the BIOS and the CMOS. The BIOS (Basic Input Output System) contains basic information allowing the motherboard to communicate with other hardware devices, in the absence of any software operating system. Amongst other things, it also contains the default motherboard settings. The BIOS is stored on a ROM (Read Only Memory) chip on the motherboard. As the name implies, this chip is read-only and can only be written to by ‘flashing’ the chip with updated software. More importantly, the BIOS chip is non-volatile memory, meaning when the power is turned off the data on the chip is retained. The second chip in which we are interested is the CMOS (Complementary Metal-Oxide Semiconductor). The CMOS memory is writable and contains the configurable BIOS settings (e.g. date and time, CPU speed). However, this memory is volatile and will only retain its memory so long as the chip is supplied with power. Fortunately, CMOS chips consume a very low level of power, so when the computer is off, a small battery on the motherboard (called the CMOS battery) is sufficient to provide enough power to keep the settings on the CMOS chip. If this battery goes flat, then when the computer is off no power will be supplied to the CMOS chip and thus all the configured BIOS settings will be lost.

When the computer boots, it looks for any BIOS information on the CMOS chip. If none is found, it will (as you have found) either revert to factory default settings or ask if you would like to enter setup to configure the settings. This is what happens to your computer when cold booting. The reason the problem doesn’t happen when rebooting the computer, is because the machine is already powered-up and the BIOS settings you configure on the initial cold boot are still held in CMOS memory because a restart does not power-down the machine. So, after this relatively long-winded explanation as to why the problem occurs, to fix the problem you will need to buy a new CMOS battery. These are available from most computer stores. It seems that a poor CMOS battery came with your motherboard, since the battery should last for more than two years! However, the battery lifetime greatly depends on the quality of the battery. When buying a new battery you should write the make and model of your motherboard (or better yet, take the motherboard manual to the computer store) to ensure you get the correct battery for your motherboard. If you don’t have the manual, you should be able to find an electronic copy of the manual on the manufacturer’s website.