1. How does a computer go from off to login?
Booting a Computer
How does a computer go from off to login?

2. Power On
When the power button is pressed the motherboard sends a signal to the PS_ON pin of the P1 Connector (Green Wire).
The chipset holds all devices in reset until PWR_OK is received from Power Supply
CPU Initializes and loads a program from BIOS which runs the POST and then loads a boot loader from the Master Boot Record

3. The Master Boot Record
The MBR is found on the 1st sector of the 0th Cylinder, using the 0th head
MBR Contains Master Boot Loader and the Partition Table

4. Master Boot Loader Looks at Partition Table for boot partition
Calculates physical location of FS Boot Sector
Loads FS Boot code to memory
Transfers control to FS Boot Code

5. Different Boots for Different OS
MSDOS v1 to 7: IO.sys using msdos.sys
Win95/98/ME: IO.sys using msdos.sys
WinNT/2K/XP: NTLDR using boot.ini
Vista/7 : Boot Manager using B