Sounds like you have a proprietry motherboard (e.g. Gateway or other big brand name) as the key for entering the bios is not always displayed on these. Common keys are DEL,ESC, F1,F2 or F10. If none of these work however, try unplugging the harddrive (power lead & ribbon cable). This sometimes forces the BIOS to come up.
If this doesn't work, you could clear the BIOS on the mobo (usually jumper near the battery, often JP1 or JP4. Sometimes it will be a dip switch). This may also force the system into the BIOS but beware as you will lose all the settings.