Here's a partial list of ways to access the built in boot menus of different systems. Press the appropriate key when the first screen shows up after you turn the computer on.
HP/Compaq - Esc (starting to use F9 also) (F10 will usually access BIOS)
Dell - F12 (F2 will access BIOS)
Gateway/eMachines - F10 (moving to F12 with newer models) (F2 will access BIOS)
Acer - F12 (not always) (F2 will access BIOS)
Toshiba - F12 (older systems will be different)
Sony - usually will boot to CD, may ask you to "Press any key to boot from CD"
Asus - Esc (F2 will access BIOS) F8 on some systems (mostly older). Tab will display the boot screen
Samsung - Esc (F2 will access BIOS)
Nokia - F12 (F2 will access BIOS)
Lenovo - F12 (?Del wil access BIOS)