log-in password not recognised in Leopard

I upgraded the system in my iMac 20 from 10.4.11 to 10.5.1, and now can't log in as my password isn't recognised. I've booted from the system disk and used utilities to change the password, but without success.
Other forums suggest a system upgrade might fix it and that it has something to do with keychain protocols, but I can't access the internet for this as I can't get my iMac to work!
Upgrades to two older powerbooks were quite successful, and without problems

G'day and welcome to the forumns.

Sounds like a clean install coming up, which is better than upgrading. Boot from the Leopard disk, go into Utilities and run Repair Disk to see the hard drive is okay, then erase the drive and format Mac OS Extended (Journalled) Go back to the Installer and try again.

Keep us posted?