Gustavo Duarte, 3 articles on the boot process :
Motherboard Chipsets and the Memory Map : This one also explains why a 32-bit OS like XP cannot see the full 4gb of ram even if you paid for it.
How Computers Boot Up : Including the MBR and why it's critical.
The Kernel Boot Process : Why it's bad to delete files you don't know what do. :)