Restart while holding down the mouse button. The CD drive should eject. Put your OS CD in and restart again holding down the “C” key. That will boot off the disc.
At that point I’d find the disc utility and repair the permissions on your hard drive. Then look for the “Startup Disc” selection in the menu and switch back to the internal drive and restart.
If THAT doesn’t do anything restart the computer. AFTER you hear the boot up chime (NOT BEFORE!) hold down the shift key. That goes into Safe-boot-mode. If that works try to re-install the update.
If all of this fails, do you have access to a 2nd Mac? You may have to hook both computers up with a firewire cable and restart your broken machine while holding “T” down. It will become a firewire drive on the 2nd Mac’s desktop. Open it up, get all of your files off and then boot it up from the CD again, erase the hard drive and reinstall the OS.
Before you do that, someone else here may have an idea. That would be a last-case scenario if nothing else works.