Grey screen, spinning wheel

While I was doing the software update to 10.4.4, I
was using Internet Explorer (god only knows why).
Explorer crashed and froze the whole computer,
nothing would work (just spinning color wheel). Shut
down the computer with the power button. Now I can't
restart it (yes, this happened in the middle of the
10.4.4 install). My hard drive is backed up on a
second internal hard drive, which should not have
been affected by the above events.
1. What is the best course to take now?
2. How do you start up from a CD on a G5 (I can't
even open the door for the slot)?
3. Is there a way to get it to start up off my second
hard drive?

to eject the CD:
Hold down the mouse button at power on. Drive should eject. insert your installer CD. Close the door. Restart. Hold down the 'C' key to boot from the CD.

To start from a different drive/OS:
Hold down the option key at poweron. Keep holding it down until you get a blue screen. you'll be presented with a screen with all recognized bootable drives. Select one, then click on the arrow that points to the right. (Clicking the circular arrow rescans the bus) This screen should also recognize bootable CDs.

If the installer was actually installing files, it is poossible that you have mixed OS version files.
Since not every file of the OS is updated during an update, you may or may not be able to recover the OS on your original boot drive, depending on what files were updated before the system crashed. (ergo, you may have to start clean with 10.4.0)
You may also see a screen with a broken folder icon - (corrupted OS)