While your mac is starting you can start with some option. Here you will find a list. To use those key combination you need press those before starting your computer.

CMD-OPT-SHIFT-DEL.... doesn't boot on internal disc but is trying to boot from external support (or CD) 

C.... starts from CD

D.... force internal disc to be the boot disc.

N.... Press until 'Apple' loge, boots from a network drive (using BOOTP or TFTP) 

CMD-OPT-SHIFT-DEL-# (#= SCSI disc).... for oldtimers, starts from SCSI drive

CMD-OPT-P-R..... Resets the PRAM (Internal memory of the computer, all your preference settings will be deleted)

SHIFT.... deactivates all extensions

CMD-OPT..... (just before the finder is starting) resets the desktop

CMD.... start without virtual memory

OPT.... allows you to choose between all the system available, including network

Thats all....

Related article:

• Shortcuts in the finder