Sure, you can tell a lot about people from the way they look. But to find out what makes 'em tick, you need to do some serious psychoanalysis. Your enigmatic PC is no different. Beneath its colourful screens and fancy pull-down menus lurks the BIOS -- a bit of software that acts as the subconscious of your PC. And just as your subconscious controls your behaviour, your PC's BIOS exerts a powerful influence on the machine's performance.

Here's what you need to know to gain the greatest insight into your PC's hidden psyche.

BIOS -- short for Basic Input/Output System -- is the low-level software that translates instructions and data from the language of the people-friendly commands that Windows (or DOS, or Linux, or whatever operating system you're running) uses into the cryptic language that controls the chips in your PC. The BIOS code resides in a chip that's located on your PC's motherboard, so it's not easy to replace.

To properly govern the behaviour of your computer's chips and devices -- including hard drives, memory, parallel ports, and a great many others -- the BIOS needs to know what types of devices your PC contains and (where you have a choice) how you want them to function. And it must have this information before loading the operating system. To make that possible, your PC stores the relevant settings on a separate memory chip known as CMOS (which stands for Complementary Metal-Oxide Semiconductor). The program used to access and change BIOS settings is called the CMOS Setup program.

This program has settings that both beginners and veterans will find useful. You can do things as simple and obvious as adjusting the time and date maintained by your system or as complicated and obscure as altering the number of "wait states" your PC uses to coordinate the flow of data between the RAM and CPU. And since some settings here can override the corresponding settings in Windows -- for example, to assign a specific IRQ to a serial port -- the CMOS Setup program is the first place to look when you aren't able to resolve a hardware conflict in Windows.

Many PCs come with a motherboard manual that describes the PC's CMOS Setup program, setting by setting. If yours doesn't, check your PC manufacturer's Web site for documentation (sometimes PC makers customise the BIOS for their systems) and then the motherboard manufacturer's Web site.