Cannot change things.

Open up the system prefs and then look at the users tab. Are you still listed as an admin or did it bump you to standard user?
If you're listed as admin, you need to boot off the leopard disc (put it in, restart, and hold down c). In the utilities menu, there's an option for password reset. Select a new password and restart and you should be on your way.
If you're listed as a standard user, and there are no other admin users on the computer, the only way around that I've found (unless you have the root administrator login enabled) is to do a clean reinstall.

This all happened to me when I did the "upgrade" option and it was the only way I could get around it. Maybe someone else has a better answer for the standard user part though.

 

Umm... you should be able to put in the Leopard disk, and then from the menu up top select restart. It shouldn't be asking for a password to do that. If it is, I'd put in the disk wait for it to mount and then just hold the power button till it turns off. Wait a while and then turn it back on while holding down C.