Boot Process explained in simple steps:

A Computer is designed to start in a predictable way from the moment you press the Power On button until the moment the operating system loads. The list below outlines the steps.
1. Power good signal is sent to the CPU -> When you press the Power On button
2. CPU looks at ROM for basic instructions (BIOS)-> When CPU receives power good signal
3. System BIOS loads
4. BIOS initiates Power-On Self Test (POST)
5. POST checks RAM and then Video. If either of these have a problem, there are various beep codes. -> Typically, a procession of long single beeps for RAM; one long and two short for video. Motherboard documentation contains beep codes
From this point forward, errors are reported with text messages displayed on the monitor.
6. When RAM and Video pass the POST test, a single beep occurs. The single beep exists simply to indicate that the diagnostic speaker is working. A malfunctioning speaker will prevent audible beep codes. -> You will begin to see text on the screen. The rapid numbers flashing indicate an in-depth RAM check. The screen will indicate the BIOS manufacturer and version number.
7. POST then checks keyboard. -> If an error occurs, a text message generally displays the on-screen
8. Legacy and then Plug and Play devices are identified -> The data gathered is then stored on the CMOS chip
9. CMOS data is queried against new current configuration data. Drives spin, lights flash, and sounds are heard. -> If there is a problem with the CMOS battery, you will get a text message.
10. Finding no major hardware errors, BIOS turns the process over to the boot loader.
11. The boot loader learns the boot sequence from the BIOS (e.g. A: C: CD-ROM, etc.) and looks for the Master Boot Record (MBR) on that device.
For hard disks, the boot loader looks for a partition table. The partition table will have a pointer to the MBR on the primary, active partition
12. The MBR contains the first file needed to start the operating system (IO.SYS in Windows 9x, boot.ini in NT).
13. The whole process is turned over to the OS and you see splash screens, etc.