Cannot Create a User Account on Mac OS X 10.5.8 Leopard
You will need an original OS X install disc for a computer that old if you can't get it to start up and get to the desktop.

Reinstall the original OS by following a guide such as this. Re-initialize the disc and create a new administrator account. 

Then you could use "Software Update" that is built-in to the OS, to update to a newer version of the OS if you like.

Good Luck!