The boot process of a computer is a bit complicated. The first software to be run in a PC is the start up program which is stored in the BIOS. The start up program contains instructions that tell your computer how to boot or start up. After this, POST (Power On Self Test) is run or actually initiated by the start up program. POST is a self diagnostic program used to check for the presence and functionality of the major installed hardware. If a hardware is found to be missing or not functioning, an error message is displayed on the screen or issued in form of beep codes, or a combination of both.
After everything is satisfied to be okay, the BIOS looks for the Master Boot Record (MBR) - a boot sector used for booting or starting up your PC. The boot sector then loads the operating system in memory where then it is executed. Then, you can be able to start your application programs and user data/files.