Can't boot by Firewire Target Disk Mode etc

I have a bit of a problem: I cant access my Mac because I lost the password. I recently changed my passwords on several macs, including an old PPC PowerMac G5 from ca 2005. I run 10.5.8 on this and it works flawlessly. I need to use this machine for important work.

So this is what I have tried so far:

1) Tested with all known passwords I have used in the past.

2) Tried to boot from a 10.6 install DVD

3) Tried to boot from just C and later Command C

4) Tried a different install DVD 10.4 (I dont have the original 10.5 DVD)

5) Tried to use T and Target Disk Mode from an iMac with Lion. (disconnected all other FW disks etc.)

6) Tried different Firewire cables

7) Tried different Firewire connections.

It starts obediently and show every sign of Mac-Happiness. But I simply can't get the darn thing to boot. I think it normally should have booted using 10.6?

Any ideas, folks?

Thanks.