My computer froze while adding an OSX.3 update and now when I start it up it doesn't complete its startup sequence. I hold down C key to force startup from the cd but it just keeps trying to start up from the main hard drive. Is there a key sequence I can use to force the computer to go to the select a startup drive window so I can reinstall OS 10.3 to get back in business?

my computer shows message to insert disk that i have'nt got the screen shows a square with a with a cross in it and can't get it to start

how can i get the computer to start without a start up disk
and what are the key board command how can i get them