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

its possible change the startup for two os but not acces the mac os x.? by the way nice forum...

Try pressing Command-Option-O-F when the system boots, you will get dropped into a DOS-like prompt, then type mac-boot then hit enter.

Try pressing the Alt (also known as Option) key at start up and try selecting the disc from there.

I am not sure what you are trying to accomplish, but if you hold the command and s key during startup, you will be brought to the unix layer of the system. Make not mistake that you are in dangerous waters at this level. You should not go here unless you have a strong understanding of Unix as you can completely destroy your Mac OS software.
It sound like you attempted to upgrade and it failed. It does happen, for many reasons. Now, do you have 10.3 (panther) or were you updating to 10.6.3 (Snow leopard). Big difference. Lets go with SL. Insert your original disk that came with your system and reinstall your system. Go to the apple site and download the your updates. SN is now up to 10.6.4 so what I would recommend is get the combo "Mac OS X v10.6.4 Update (Combo)" file as it is a complete system update.