The second you switch the computer on it will start running software. The first thing it will run will be loaded from the ROM. The program located in the ROM will -

The POST essentially just checks for the existance of hardware and tells them to activate. On a normal system it will display the results on the screen. This is sometimes hidden by a logo or image which can be disabled from the BIOS settings.

Settings stored in the BIOS tend to be stored in a special area known as CMOS (Complementary Metal-Oxide Semiconductor ). This is RAM which will store any changes you make to BIOS settings. It will be powered by a small battery which allows it to save data when the computer is switched off.

The boot file will contains parameters needed for the operating system to load. It could simply start the OS off or give a choice of what OS to run (if more than one is installed). It also may give key board commands to allow the editing of settings. This is definable by software so will differ from OS to OS.

Normally the OS is stored on the hard drive but most computers will check the CD-ROM or even USB drives to see if a boot file can be found on these. The reason for this is because most OS are installed off CD and by allowing the boot from these devices it allows the user more control. Which devices the computer will look for the boot file can be set in BIOS.

A OS will then start to further define and extend on the BIOS. It will also define how files will be stored and how much memory is available to programs! NTFS is the standard file system for windows, FAT is the standard for older versions of windows and DOS while EXT3 is the standard for Linux (EXT4 is also starting to be used which is an upgrade to EXT3).