1. some PC's are setup with "silent boot", you get a logo screen [not w98 logo] and no hardware detection info, no ram count, and no line "press __ to enter setup". You can turn this off in setup.
2. F1, F2, delete, are most common keys. alt-ctrl-enter, alt-ctrl-esc are on some. You can hold down any of the 1st 3 keys during initial boot, until you get a multiple beep from PC which means stuck key.
3. An ultimate way is to unplug the Hdd. You'll get an error message and usually the option "F1 to continue or F2 to enter setup". Continue means to continue booting anyway. You'll get the next error message that there isn't anything to boot from.
4. Be very carefull messing with BIOS. You can change things that will make things inoperable.
5. Usually there is a "security" option. Here is the "administrator password" for prevention of entering BIOS. "user password" prevents booting to O.S.
BE SURE you write these down, otherwise you have to find the jumper on the mobo to "clear CMOS".