Hand me down iMac at work

I'm working on an iMac that was used by a former employee. I'd like to wipe the previous account clean and make it my own (user account, keychain passwords, etc.) Also upgrading from 10.5.8 to 10.6.

What's the most efficient way to do this?

Thanks in advance!

The best and most efficient way is to use the original install media (Leopard DVD set) to boot the machine and then do a complete erase and reinstall of the operating system and applications. Once you have a clean Leopard running, you can use a Snow Leopard upgrade DVD ($29) to bring everything up to date.