MBP won't boot after upgrade to Leopard + ways to backup data

I got a problem that many of you may have encountered. I was installing an upgrade from tiger to leopard the other day, and I left my computer when the installation screen was saying that it was installing into the system or sth. After I came back the screen is already grey, with the apple logo in the middle and the spinning wheel on. The computer can never load into the system. I closed the computer using the power button and open it again, and again the system would not load. I called in the apple care hotline and they suggested a series of actions such as pressing the option button and other actions you do to boot your machine and none of them worked. The only time it works is rebooting from the CD, where it goes to the installation menu. If I choose to quit and reboot from the macintosh HD, the computer goes back to its own state of the grey screen and never loads into the system. The most important thing is, I HAVEN’T BACKED UP MY DATA YET, which I know is probably the stupidest thing I’ve ever done. (I’ve helped a few friends install an upgrade and they never need a backup of their data, I dunno why it’s me that is so messed up)

Anyways, here are my questions:
1. should I try install the upgrade again and see if they can load into the system afterwards ( I will boot from CD and then install upgrade), but I won’t be backing up my files yet
2. should I find another computer to connect to my computer thru terminal and do a drive image of my computer ( but the thing is, the drive image will contain the corrupted system which I have in my computer now, so might not work). I will then do a erase and install ( note: I lost my word installation CD so I will probably have to buy it again after doing this)

any comments or similar experience in this issue is appreciated. Thanks!!

An external firewire drive would be the go, copy Tiger via Super Duper and it will be bootable, assuming you can get back to Tiger. Try holding down the option key when booting to select the 10.4.11 or whatever as the startup drive.

Then format and a clean install. There seems to be a whole lot of problems updating Tiger - cat of a different colour perhaps?

Thanks! However, I think my system has been updated to leopard now (I tried rebooting from CD and it indicates that the system installed in my harddisk is OS 10.5). In that case, is it possible for me to downgrade it back to tiger?

Also,I heard that I can try rebooting from CD and install the upgrade again. Would doing that solve the problem, and would that currupt the data in my harddrive now? (it's not corrupted yet now)

Don't like the whole upgrade option of Leopard - a clean install fan for all OS.

To downgrade back to Tiger you need the Tiger disk, a full retail version black coloured disc, back up all your information and reformat the drive and a clean install.

Here is info on this topic:-

http://forums.macrumors.com/showthread.php?t=384582