log-in password not recognised in Leopard

I've just upgraded from OSX10.4.11 to 10.5.1 (box set) on my iMac 5,1, and on re-booting have found that my password isn't recognised. Password changes made through utilities on the install disk haven't helped, and starting from a back-up system disk and changing passwords, owner or administrator privileges, etc, have also failed.
Other forums suggest it is a keychain access issue that might be fixed with a system update for 10.5, but I can't get that without starting the iMac! Using the back-up system to boot doesn't help as I get a disk full message.

Any suggestions?

Upgrade to an older G4 power was fine.

The iMac model is 5,1
Proc 2.16GHz Intel Core 2 duo/2 GB memory

Any chance of starting the iMac in Target while connected to another Mac and run the combo updater onto the iMac HD - might help, and you need to get to 10.5.8 anyway.